You absolutely must disclose the sources of your income. If you do not have this information and you end up being audited you could be in for a world of hurt. So yes, of course we will see where the money came from in Trump's tax returns, that much is certain. The real issue is that his tax returns are (from my understanding) just a shitload of LLCs, each with their own tax returns. They also must declare the sources of their income, which in many cases will be other LLCs. You then have to go pull their returns, etc, etc.
I don't think it will be easy to track down who is paying Trump because I suspect he (and likely his counterparties) have made it deliberately difficult to do so. That's all the more reason Congress should have started this investigation years ago.
Did you already forget how the NYT uncovered tens or hundreds of millions of dollars worth of tax fraud by Trump and his father using... you guessed it... his tax returns? Like, the thing you're calling wishful thinking literally already happened.
What do you think is more likely to be wishful thinking, the fact that Trump is desperately holding on by his fingernails against any attempt to release his tax returns because there's nothing in them, or that Trump has continued with the same criminal behaviors we already caught him doing in the past? What does common sense say?