I don't think Biden is a lock. I think he is a default. It's "his turn", unless somebody better comes along. Which is a pretty low bar. Yes, Biden is in the lead with about 33%, but "not Biden" has 67%. As the race settles, more candidates will drop out. It will be interesting to see where that 67% ends up.
And I'm thinking there's a possibility that Gaffer Joe will stick his foot in his mouth all the way up to the knee, and a lot of that free range 67 will possibly wind up with Liz Warren, unless she too does the foot in mouth thing.