The polls are in, and the big winners from the first Democratic debate(s) are Elizabeth Warren and especially Kamala Harris (as I said -- don't listen to me).
Of course, it's still very early. But right now there's a four person race between Biden, Sanders, Warren, and Harris -- and a pretty steep drop-off after that.
For me, that means that the biggest loser from the first debate was not actually Joe Biden, but Pete Buttigieg. Buttigieg had been right up there in the front-runner conversation, and now he's on the outside looking in. Beto O'Rourke was in a similar circumstance, although he had already seemingly started to fade -- a lackluster debate performance only confirmed the trend. But that Buttigieg, who did not seem to perform particularly poorly (though hardly scintillating either), got separated out from the top-tier of candidates is much more of a problem.
While Sanders is still part of the Big Four, I'm not sure that this is good news for him either. He's already at near-maximum name recognition, and it's far from clear from whom he'll draw supporters as other candidates drop out. The most promising target is Warren -- but now it's more likely that Warren will last to the end, and maybe Sanders will start bleeding his support to her.
The other big mover people have mentioned coming out of the first debate was Julian Castro, but it mostly looks like he went from "complete non-entity" to part of that mid-tier group that's outside-looking-in (like Booker, O'Rourke, and now Buttigieg). Definitely an improvement for him, but not the surge we saw out of Harris.
As for Biden, I agree with Paul Campos that he's in the position of having a massive early lead that you know is going to get eaten away -- the only question is how fast. I don't think he's going to be able to hold it. But he's actually in a fundamentally different position and I'd say better position than Sanders, who affirmatively needs to grow and doesn't have a clear route to doing it. Biden just has to stem the bleeding for long enough that the clock runs out -- though I have to say, that isn't the most inspiring way to limp into a general election.