Why did so many people think Biden would inevitably "collapse"?

[Southern Senator North Carolina Yankee]

There were a number of reasons to expect that he would fall apart and he probably would have had a compelling alternative appeared that could appeal to all of his base. Each of the alternatives had some problem with part or all of Biden's support base and as long as Biden's support wasn't taken away, he remained a default choice against Sanders. Ultimately, maintaining strong black support was the key to his revival and once he dominated SC, the support that wavered surged back along with a number of others determined to stop Sanders.

A good validation for this point is the fact that Bloomberg got in. He would never have got in if he had faith in Biden's ability to win and defeat Sanders. His getting in showed a complete lack of confidence motivating him to take the plunge.