It isn't about changing the mind of Biden supporters. It is about winning the expectations game.
The whole "Joe has dementia" talking point that the Trump campaign and their conservative media lackeys have pushed has lowered the bar for Biden in this debate. All the attacks on Biden's mental status have only served to lower expectations. That is why the talking point makes NO sense for the Trump campaign. The talking point only plays with the Trump base but serves to benefit Biden when it comes to expectations.
The Trump campaign should have been building Biden up over the past few weeks. Talking about his extensive political experience, his political and policy knowledge, etc. It is about winning the expectations game, not just throwing out stupid talking points to your base.
btw, while I do think Biden's expectations are low tonight, he still has to perform well. He can't make any serious blunders. Also, typically, the first debate is usually harder for an incumbent President. Biden needs to make sure this is the case tonight by going after Trump and placing Trump on the defensive.