Indeed. Even if Joe Biden gets on the stand and admits he's really D.B. Cooper, kidnapped the Lindbergh baby, killed the Black Daliah, buried Jimmy Hoffa, and paid off Ukraine to shower his son in hookers and blow, it would change the fact that the President of the United States did something illegal.
"Whatabouta" is not a legal standard.
That completely misses the point. It has nothing to do with "whataboutism".
Trump will allege that there was strong reason to believe that Joe and Hunter Biden were engaged in corrupt acts that involved US government funds. That's a pretty serious issue, isn't it? Never mind that it's all part of Trump's twisted bizarre mind in which Trump is the center of the universe and everyone who opposes Trump is automatically a bad person doing corrupt things.
If that were true, if there really was reason to believe that the Biden family was part of Ukranian corruption, then everything Trump did would be perfectly reasonable.
So, if he is on trial for corruptly messing with US foreign policy, all Trump has to do is show that his motives were pure, and one way to do that is to show that there was reason to believe that Joe Biden's motive for demanding the dismissal of the Ukranian prosecutor was as part of a coverup of Burisma's corrupt practices.
Do I actually believe that's true? i.e. Did Trump really believe that Biden was corrupt? Yes, I do believe that. Let me emphasize something. I don't believe that Biden was corrupt. I do believe that Trump believed it. The man is a friggin lunatic, and that should have been obvious to everyone as soon as he started talking about his inauguration crowds and his "landslide" victory. Even if they didn't realize it before, that should have been a wake up call to everyone that this man is a lunatic.
But, it wasn't, and still isn't. Sad, but true. So, we are left playing politics. You want witnesses? Well, then, Trump's team gets to call witnesses, too, and their defense rests on Donald Trump's belief that the Bidens were behaving suspiciously and that there ought to be an investigation into their behavior. Surely their own testimony about that behavior is relevant. And if Biden won't testify, that gets awkward when you are saying that Trump's facilitation of non-testifying is a "high crime or misdemeanor".
In reality, I have no idea what will happen if it gets to that point. Who will even make the decision about whether Biden testifies? Is it Roberts? Is it the majority of the Senate? It's one of those, and I don't know what will happen. Presumably, if it's the Senate majority, it will go the GOP way, but perhaps not. The final verdict is not in doubt, because 2/3 is needed to convict, but Romney, Murkowski, or Collins could defect on some rules questions.