H & R BLOCK, INC., the tax consultants, say they expect their business to decline in 1978. Normally, we take no pleasure in such reports; businesses, like individuals, ought to prosper. But you've got to concede this much: What is bad news for the stockholders of H & R Block is good news for American taxpayers. If Henry Block is right, when tax time rolls around next April, at least 500,000 of these taxpayers won't need the kind of help they hired last spring.

We admit we have been rather churlish in our skepticism about the "simplification" aspects of the 1976 Tax Reduction and Simplification Act. And we suspect that the drop-off in customers Mr. Block anticipates will be due to the fact that the law raises the income level at which tax returns must be filed and increases the standard deductions. Nor do we expect the new tax form to be any easier than the one that arrived at our door last year and the year before that - unless someone at the Internal Revenue Service has worked a miracle.

Still, we welcome the news that at least some taxpayers will no longer feel obliged to buy the services of lawyers, accountants and firms like H & R Block. There is something profoundly wrong with a system that leads ordinary taxpayers, with no special problems, to believe they must hire professional help to fill out a form sent out by their government. Mr. Block's firm prepared 10 million of the 85 million tax returns filed last spring, and most of them were what IRS would consider routine returns. Knocking off a half-million of these is only a small step toward what Congress ought to be trying to accomplish. But it is, at least, a step.