You'd then be defining "successful businessman" as a subjective X-factor. But that X-factor doesn't apply here; the argument above was that, as a successful businessperson, Trump has demonstrable competence in dealmaking. If he barely beats the S&P500 (or, depending which of 5 years in the late '70s or early '80s you start counting, severely lags it), that competence hasn't been demonstrated.

This is a quagmire I let myself get swallowed in so often that its likely not healthy, but my favorite 'Trump' stories all revolve around the crazy things he is accused of that no 'successful' business person would need to do.

The best of course is the accusation that he tried to disguise his voice to convince a forbes reporter that he was richer than he was to get on the top 400 list:

http://fortune.com/2018/04/20/trump-lied-wealth-forbes-400-l...

This is actually my own pet conspiracy theory. He didn't want to release his tax returns because he's doing shady tax dodges, he did it because it proves he's not actually rich.