This is the exact point I got attacked for on the left. My take on the Biden thing is that he looks guilty as hell BUT that the laws are such that what he did is common practice. Rather than defend Biden (backing a loser IMO) the dems ought to step forward and actually try to pass legislation that would make what the Bidens have done illegal. That way they can call out Trumps corruption too.
Biden is not even among my top 5 preferred candidates... I think he would be Hillary 2.0 and his entire campaign is coasting on the fumes of being former VP, benefiting from name recognition/status quo... Nobody is excited about the prospect of a Biden presidency, he would have to rely entirely on electoral disillusionment with Trump... a coin flip at best. Even if there is no actual wrongdoing and its just garden variety nepotism/swamp, I think its a major distraction and his presence in the race just gives Trump something to point at instead of his own problems. If Biden bowed out gracefully, Trump would be put in a bad position; He'd lose his talking point distraction, and does he continue the investigation (wasting time and resources just to maintain the sham that "he was just out to fight corruption") or does he drop it completely, proving that it was merely a partisan political hit-piece? or, even more stupidly: does he start looking for dirt on Bernie/Warren/etc?
I think the problem with the Dems pursuing legislation like you suggest is the fact that Trump would just use it to vindicate his desire to investigate Biden (IE: "If there is nothing wrong with what Biden did, then why do the dems want to change the law now?"). It basically plays right into his narrative of "the other side is corrupt" and he will use it to distract from what he did, which was illegal. The Trump zealots are completely blind to even the most reasonable criticisms of Trump, and the garden variety conservatives simply dismiss his worst actions as "politics as usual"... so it's not like there is any political advantage to doing what you suggest, other than handing republicans some free ad ideas because "they aren't the corrupt ones."
(Don't get me wrong, If Biden did anything illegal, it should be looked into by our own authorities, then relayed to foreign authorities (as is supposed to happen with MLAT) and Trump wouldn't even need to personally get other countries involved, but he did and that should be a problem for anyone who isn't a partisan hack. It would be fantastic if there was a way of achieving bipartisan legislation that eliminated the corrupt nepotism/backroom dealing bs, we had congressional/senatorial term limits and we could prevent these assholes from the revolving door lobbyist positions... but when you're up against people that will deny facts and reality to win, it's not a very effective strategy.)