"Everyone cheats on their taxes, not just Trump" functions as a defense, to the degree that it does, because we have a political discourse that determinedly elides all distinctions of power or class position in discussions of personal conduct
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I'm also not convinced that it is, as a matter of fact, a true defense. Many wealthy people engage in tax avoidance - legally reducing one's tax burden as much as possible. What we have of the records strong suggests Trump engaged in tax evasion - illegal tax cheating.
There's an argument to be made for trying to limit tax avoidance too, of course. Speaking broadly, complicating the tax-code with 'loop hole plugging' seems less effective than radical simplification. But it is important not to conflate avoidance and evasion.