The only reason why you're treating "What if this means that Joe Biden is corrupt too?" as a reasonable inference and not an arbitrary speculation is because Trump aggressively pushed the narrative that Biden was corrupt and involved in his son's activities.
That actually isn't the reason at all. The primary reason is that someone directly involved in the grifting with Hunter Biden named Joe Biden being involved. Maybe he is lying, I am sure we will find out eventually.
I think right now Hunter is only under investigation for Tax issues. Something about not declaring $400k. He doesn't appear to be investigated for any other nefarious deeds other than tax evasion. But for all I know, he'll get pinched for not paying taxes as alleged. Which doesn't seem to have anything to do with his Dad.
As for the other allegations that were previously put forth by the Guiliani laptop affair and the Tony what's-his-name business partner who implicated Hunter and the elder Biden in some sort of 'grifting', that was somewhat vetted by at least Fox and more specifically Tucker, and they didn't find much if anything. There might be something there, but that doesn't seem to be the current focus of any investigations as far as I'm aware.
So yeah, Hunter pretty much seems to be a first class fuck-up willing to skate on the family name. But I'm unclear as to if there's anything more than that in the mix. Presumably it will all come out in the wash.