Trump is basically right here.
He was worth an estimated $3 billion by Forbes. Even if this is a $400 million Forbes did not previously know about, and even if the IRS catch him for another $100 million in fraud, he's still eye-poppingly rich to the tune of $2.5 billion. So as bad as having 15-20% wiped off his net worth is, in context it's easily sucked up. I'll bet he didn't want anyone to know (see below) and is angry that they do, but seeing as they now do know, brazening it out is the best tactic.
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The bigger damage for Trump is that the tax revelations suggest he's not a good businessman. So many of his investments are poor quality - for instance one of his golf clubs he bought for about $150 million, invested about $200 million, and it's now only worth $150 million and loses money... and this is not a rare occurrence in his portfolio. He's got some properties mostly from his early career that make a huge amount of money, and a load of hotels and golf courses mostly from his latter career haemmorhaging money. I think the reputation hit is probably much more embarrassing for him. Had it not been for the merchandising, licensing etc. from The Apprentice, his finances would be in a terrible state.
The second issue is that he's a champion tax avoider. In any normal world, this should shatter the reputation of a politician who claims to "drain the swamp" and govern for the average Joe against the elites, as it proves he is one of these elites exploiting the system to the max, and he hasn't done a damn thing about these dodgy arrangements whilst in office. It's evident that experts are scouring through these tax returns. I see a lot of descriptions like "highly unusual", but very little in terms of "clearly illegal". That is a scandal.
If we go back to the idea he's worth $2.5 billion, now let's consider how he's used the tax code to write off losses. That was about $1 billion in the 1990s. It appears to be hundreds of millions more from the late 2000s to now. Subtract that, he'd be worth what, $1 billion? Bearing in mind how much money his dad left him over the years and factoring in inflation... has he actually made any money at all?
In other words, it seems to me that the US tax code enables failure. Bad businessmen are squatting on wealth and productivity, and when they should be competed out by better businessmen, are able to protect themselves and continue wasting those resources. That is an even bigger scandal.