BECAUSE A substantial part of the population depends on Social Security for much or all of its income and millions more are counting on its help in the future, policy-makers have traditionally spoken about the system's future in only the most soothing of tones. Fashions change. The new style of official discourse -- reflected in our letters column today in a communication from Robert J. Myers, deputy commissioner of the Social Security Administration -- poses the threat of total system collapse unless major reform is undertaken.

Social Security, Mr. Myers tells us, is "teeteering on the edge of bankruptcy." He takes issue with a recent assertion of ours that if worst comes to worst -- something we don't recommend -- checks would continue to be mailed out, but they would be short a few cents on the dollar. Not so, Mr. Myers informs us; checks in the full amount would be mailed out, but they would be a few days late. All right. We think that mailing the checks on time but about 6 percent short would cause less disruption than the financial equivalent of delaying them for two days, but the point is the same. Because Social Security is financed on a pay-as-you-go basis, payroll taxes would still flow into the misnamed "trust funds," and checks would still go out. That is something important for the people waiting for them to be sure about.

This does mean that Congress can sit on its hands and do nothing.It is true that if the economy does as well as the administration hopes. Social Security could continue businesses as usual with just a little short-term borrowing among its accounts. More likely, a cash-flow problem will occur as early as next year. The main reason for this shortfall is that wages have not kept pace with prices in recent years and that, when automatic cost-of-living adjustments were added to the system in 1972, the formula used to calculate initial benefits at retirement was defective. As a result, the proportion of wages that Social Security replaces for the average earner retiring at 65 rose sharply after 1975 from the 42 percent intended by law to almost 55 percent this year.

Amendments adopted in 1977 will gradually correct this problem by 1986, but in the meantime the system will be under considerable strain. To be on the safe side, some adjustment in either revenues or benefits on the order of about $10 billion a year, or roughly 6 percent of benefits, is necessary to avoid the certainty unacceptable and entirely avoidable possibility of either or delayed checks. Short-term borrowing from general revenues would provide the necessary cushion. If the administration won't buy that, some adjustment in the cost-of-living adjustments seems the fairest method since overindexing of benefits -- particularly at the time of retirement but also, to a lesser degree, thereafter -- has been the main cause of Social Security's troubles.

Whatever choice is made, one useful outcome of the current debate would be for the two kinds of exaggeration -- "all is well" versus "man the lifeboats" -- to cancel out and produce the kind of sensible discussion that can best guide the periodic course corrections needed to keep our most important social program afloat.