I do think there's a decent chance Biden will eventually start to fatigue and potentially drop down despite being favourite, and I think he'd be a self-evidently terrible choice for the nominee at this point, but I do think there's a fair bit of confirmation bias going on here wherein people who don't want Biden to be the nominee are forming arguments against him eventually winning the race on the basis of every small eventuality which may lead to him losing, without applying the same pessimism to other candidates.
As I say there's a decent chance he won't last the distance but as it stands he's undoubtedly the favourite. Even if he struggles in the long-term it seems he's past the point of a catastrophic early plummet like Jeb in 2016 and it's clear his role as VP during the Obama era stands him in somewhat decent stead with a lot of voters.