It will come down to the economy and how they manage to frame the Coronavirus handling I think, yeah. Biden has been in national debates before and done fine. I don't expect him to fall on his face. The talk about mental decline is going way overboard.
Other threats to him are his record on things like the Iraq War and drug war being terrible and how that can sap enthusiasm on his side. He also is not the most economically or socially to the left in general and will have pissed off some Bernie fans.
And then you have the Coronavirus threat forcing a super traditional establishment candidate to run an agile, potentially mostly online campaign that can adapt to a world changing at light speed. I feel like he may come off like McCain in 2008. Generally respected by voters, but not seeming to be able to keep up with the times. I would not be surprised if his VP pick is some sort of stunt too.
In the end, the problem is Democrats went with a candidate who essentially has a spiritually conservative message. It's not the literal slogan of the Biden campaign, but everyone can read it between the lines, "Let's go back to the Obama era. We had a good status quo but now things changed for the worse and we want to go back to the way it was in the past."
The problem is in the middle of a crisis a conservative trending electorate seems very prone to sticking with the actual conservative incumbent to me. They seem like the types to be persuaded by a, "Don't change horses in midstream," approach. Were we ready for a pandemic under Obama? As far as I can tell, fuck no. Maybe the response would have been handled a little better, but the masks weren't stockpiled then either, right?
COVID-19 has changed the world and nobody can deny it. I don't think it's time for a nostalgia candidate. Democrats are in the very unenviable position of having to root against recovery to some degree now if they want to make beating Trump priority #1. If Trump does a good job with the economy going forward without killing people his approval will go up and people will be grateful.