It will not be as hard as anybody thinks to replace Johnny Carson, because Johnny was never really all there. He only made the impression of making an impression. Johnny, we hardly knew ye - and so much the better for us. Carson's essential innocuousness helps explain his 17-year endurance record and our long-running appreciation of him.

As Marshall McLuhan said, "High-definition" personalities cannot last long in television. Johnny is about as low a definition as you can get. If he got any lower, he wouldn't even register as an image on a TV picture tube.

The Johnny apparition we see on the screen has no style, spine or spleen; we can connect the dots and make of him what we want. He is always bright, but never brilliant. Would we be able to stand the wittiness of a Noel Coward or a Dorothy Parker, night after night, in our bedroom? No.

NBC and Carson still have not announced their settlement of Carson's current tiff; it is possible, if not likely, he will remain as host of the "Tonight Show" beyond the end of the year. But that doesn't mean it is too early for the public and press to start speculating about possible successors. Printed rumors about people like Burt Reynolds or Bill Cosby being candidates for the job are patently nonsensical.

Reynolds only hosts the "Tonight Show" occasionally to plug his films and would hardly be interested in the job. Cosby is a comedian who, when strapped for something to say, begins go shout. He would drive us all mad in the course of a week; two nights of him are pushing it.

From Hollywood comes word that a prime candidate for the post is a 31-year-old comedian named David Leterman, part of the repertory company on Mary Tyler Moore's first flop variety show of the season. Letterman did the warm-ups for the show and entertained the audience with stand-up comedy during breaks between shooting, while sets were moved around.

He has also appeared as a comic on the Carson show and guest-hosted a couple of times. But the most that can be said for him is that he is affable in a bleak sort of way; he is a pale Johnny Carbon, and Carson is already pale enough. "Low Definition" does at least imply that there is some definition there somewhere.

Letterman's manager, Lee Cohen - one of those surly William Morris Agency types - said yesterday from L.A. that NBC has not asked Letterman to be the new Johnny Carson, but then they'd hardly be at that stage yet with anybody.

"If Johnny Carson leaves, David Letterman might be a contender," said Cohen. "However David has his own deal with NBC for develoment of prime-time shows and other projects." One Hollywood sage notes that Letterman has the same "Midwestern - kicking kind of humor" as Carson, but NBC didn't look for a Jack Paar clone when they replaced Paar, so why should they want a Cloney Carson?

Other names mentioned as possibly Carsonogenic include the irrepressible - though we should keep trying - Tom Snyder, who recently announced a one-year prime-time deal of his own with the network and is not a likely candidate; Steve Martin, who can make much more money as a concert comedian and record artist; Chevy Chase, who has said repeatedly he doesn't covet the post and wouldn't last six months; and Phil Donahue, the take-charge syndicated talk-show host whose sense of humor is about as lively as David Eisenhower's.

Then there is a model contender: Richard Dawson, host of the ABC game show "Family Feud" and by weird coincidence the guest-host on ""The Tonight Show" last Friday night, right after the Carson story broke. Carson was a game-show host, too, when NBC picked him to host the "Tonight Show" in 1962. Dawson has made such prime-time appearances as "Bizarre," a comedy pilot ABC aired but didn't buy for next season, and on high-rated "Family Feud" nnighttime specials.

Dawson has the ability to do standup topical comedy but also to sit down with guests and appear to be genuinely interested in what they are saying. He could be highly satisfactory as the host of "The Tonight Show."

"Oh yeah, I'd love to do that show." said Dawson when asked, earlier this week, at his Los Angeles home. "But nobody can do it until Carson decides whether he's leaving or not, because that show is like a suit for him."

Dawson got a larger-usual ovation for a guest host when he walked out Friday night. After the applause subsided he told the audience, "I've already lasted longer than Ken Norton in his last fight." Dawson was up until 2:30 a.m. answering congratulatory phone calls from friends. His performance was not his best or most relaxed, however; in addition, he was poorly lit and subject to the whims of Carson's personal director, Bobby Quinn, who always knows when to cut to Carson for a reaction but appeared reluctant to help Dawson look as good as possible.

"I had a ball," says Dawson of the appearance. "I really did. I was very nervous beforehand, but the moment I walked out there, I had the time of my life." Dawson is easy to take but engaging enough not to be dull and, he says, though not campaigning for the job, "I think I could bring people to that show who normally wouldn't watch it."

"That show," after all, is not attracting as many people as it used to.

A five-year comparison of "Tonight Show" ratings reveals a steady erosion and loss of audience to ABC and, especially, CBS, which has buffeted the program with such ironic counter-programming as old episodes of NBC's "The Rockford Files." In the recent February "sweep" ratings - during which Carson appeared on most of the programs telecast - "The Tonight Show" won over CBS for the month by only one share point, 28 to 27; ABC got 23 percent with its unsavory line-up of violent crime shows (including another NBC oldie, "Police Woman").

"The Tonight Show" is ripe for remodeling. It's true that in the first 20 minutes each night, Carson, with announcer Ed McMahon as his foil, can still be nourishingly amusing. With Paar, the keynote of the show was unpredictability; Carson seems to have made just as big a draw out of predictability - ritualized, formatted comedy "bits" that are funny partly for their dogged familiarity.

In pursuit of laughter, Carson has always been more of a borrower than a lender, from his earliest TV days when he mimicked the delivery and timing of his idol, Jack Benny. From Jonathan Winters' Maude Frickert Carson borrowed "Aunt Blabby." From Steve Allen's "The Question Man" he took, "Carnak, The Magnificent." From Fred Allen's Mighty Allen Art Players he took the Mighty Carson Art Players. From Jackie Gleason's immortal Reginald Van Gleason III he took the vocal style he uses as "Art Fern," the host of "Tea-time Movie."

Carson is now and always has been about as original and distinctive as yesterday's coffee.

And although there are many nights of merriment and even hilarity to be grateful for, in certain ways Carson ruined "The Tonight Show" that Steve Allen and Jack Paar had built. When Carson moved the program from New York to Los Angeles, it was a strategic, symbolic and highly detrimental migration. All the wit went out of the show and the West Coast pod people flooded in.

In airport acronym parlance, Carson put the show firmly on the LAX-LAS axis; one of the reasons he couldn't do the show live, as he wanted to a couple of years ago, was that so many of his guests are simultaneously playing Las Vegas and couldn't make their shows and Johnny's too. About the only political guest Carson tolerates now is Mr. Granola, Jerry Brown, who sails in on the same cloud as Orson Bean and Joan Rivers.

Paar is remembered for fits of temperament and rancor. These were a very small part of his program. He was a genuine conversationalist who brought out the best in a wide variety of raconteurs and cast jaundiced eyes at the pretty but empty. When Carson entertains a guest like Robert Morley or Orson Welles, he hasn't the talent to get them going on an anecdotal joy ride of their own because he is too busy plotting his next laugh.

In person, Carson is a cold, glum and suspicious man. He has been knwon to refuse the use of tape recorders during interviews because he fears the tape will be released as a commercial record (someone did try to make, with his permission, a "Tonight Show" LP and it was a colossal commercial bust). On the air he appears a man of no convictions - other than that the horse is smarter than the pig - or backbone, except that he will attempt stunts and let big insects crawl up his arm.

We will be losing a great deal when Carson leaves "The Tonight Show," but it shouldn't take more than three nights to recover and forget him completely. CAPTION: Picture, Richard Dawson