Listing a bunch of non-sensical hypotheticals isn't very compelling. Vlad was never going to be a league average 3B. This was obvious to anyone who watched him play third. His range was bad, his glove was bad, his first step was bad. Getting in shape would help.maybe one of those. Not sure how you can compare to Bichette but ok, I'll oblige.
If Bichette had to move off SS because he wasn't good enough and needed to move to third, then he'd be just as valuable. Someone else would play short. Not a big deal.
I believe the issue stems from a group of people who, via extreme homerism, had this strange perception that Vlad was essentially going to morph into a different person and go from one of the worst 3B in baseball to an average one. I remember watching Vlad make plays at third what were like 85%+ outs and people fawning over him for making them. It's even part of the reason I'm glad he's moving to first.
I'm not going to address the Biggio getting fat thing because it's really dumb and the answer is obvious