In his latest New York Times column, Paul Krugman explores some of the reasons why Donald Trump is claiming he can't release his taxes (because of course, he can), and argues it is most likely he isn't anywhere near as rich as he claims to be:
Mr. Trump’s excuse, that he can’t release his returns while they’re being audited, is an obvious lie. On the contrary, the fact that he’s being audited (or at least that he says he’s being audited) should make it easier for him to go public — after all, he needn’t fear triggering an audit! Clearly, he must be hiding something. What?
It could be how little he pays in taxes, a revelation that hurt Mitt Romney in 2012. But I doubt it; given how Mr. Trump rolls, he’d probably boast that his ability to game the tax system shows how smart he is compared to all the losers out there.
So my guess, shared by a number of observers, is that the dirty secret hidden in those returns is that he isn’t as rich as he claims to be. In Trumpworld, the revelation that he’s only worth a couple of billion — maybe even less than a billion — would be utterly humiliating.
The revelation that Trump has been lying about something won't do much to harm him given he lies on literally a minute to minute basis, but Trump's wealth is essentially his only real claim to fame. Of course should Trump decide to release his tax returns and concede he is in fact a medium rich asshole and not a mega rich asshole, he'll likely lie about that too (the government underestimates the value of his brand and so on). But Americans love him because he appears to be a mega rich asshole who takes what he wants from whomever he wants, and won't apologize for it.
Should it turn out that he is significantly less wealthy that he claims he is, it undermines the image he has carefully crafted over the years -- a chip in the armor that will be exploited mercilessly by his ever growing list of mortal enemies.
So basically, don't expect Trump to release his tax returns any time soon.