You confuse what Reagan did and what the Dims did later on. By your logic we shouldn't do anything at all to the tax code now because at some point in the future the Dims can undo it.
But he should have realized they would not keep their end of the bargain. He should not have let them take away the deductions.
Actually, the real problem is spending. I like the idea of holding the spending to a certain % of GDP, but I wonder if that would even work. If the American people really wanted smaller, less expensive government we would have it by now.