The argument is that he would have been an already-there stand-in solution to whatever it was we did for the past 2+ years. If the argument is that he's unplayable in the playoffs... well, we had Ed Davis, Tony Bradley, and Derrick Favors. All of those were unplayable, and the defense suffered. Whiteside is much more well-rounded. The argument is more about prior years and how we botched this whole thing by not looking for the right type of guy (such as a Whiteside) to backup Rudy, and instead pretended that Favors was a rim protector or poor man's Rudy.
With the exception of McGee, all of those guys were looking for contracts in the league. And again, if we didn't have Whiteside right now we'd either being saying he sucks or saying some variant of "he'd never come here." The beauty of viewing things through this lens is the belief that we're absolutely preforming at the pinnacle of our capacity from an executive and managerial sense, and there's absolutely no way we could improve on status quo because if there was we would have already done it, and our inaction is therefore proof that it can't be done.