The issue (before Thursday) was that the anti-Biden wing of the party couldn’t really demonstrate that the alternatives had any better shot of beating Trump at all. All the reasonable alternatives backed out of any major 2024 challenge precisely because they are young and can wait until 2028 (or even later!) to make their push. Once Biden announced for 2024, it took a lot of the momentum out of finding any Democratic challenger.
There is an assumption, though, that the person Democrats are nominating in 2024 is the same man who beat Trump in 2020. I don’t think we can make that assumption anymore. Biden had his chance to demonstrate that and blew it. He doesn’t get that many more chances. If he continues to blow them, then Trump wins by default. No Democrat wants that.