I thought Biden was too old in 2020. I know he's too old now. But it doesn't matter. Obama 2.0 isn't on the sidelines warming up and ready to come in. None of the possible alternative candidates offer a substantially better chance of winning (all are polling just about even with Biden against Trump, well within the margin of error) and none but Harris would even want to try because starting a campaign from scratch with 3 months to go would be a nightmare.
Just like in 2020, our candidate is "not Trump." Even if Biden steps aside for Harris or if the party somehow nominated and coalesced behind another ticket entirely, the candidate wouldn't be Obama 2.0, it would still be "not Trump." Biden was the acceptable old white guy in 2020 who couldn't even really get the message across then. Yes he's worse now, but not really even that much worse. His decline is being magnified and everyone is doomposting because most people wanted to believe for the past 3.5 years that there was no way that Trump could possibly get re-elected and now they're finally confronting reality. But this was always going to be a close election, just like the last one, for numerous structural and social reasons that have nothing to do with who the Democratic candidate is.