Sure looks that way,

Nothing would ever change a Sanders voter's mind ...

Take a look at that top graph, and ignore the polling before around May (because there are too few options to draw many conclusions, IMO). If you do that, then the only candidate who is moving up is Buttigieg (other than the blips for Harris and Warren). The others are flat (Sanders) to down (Biden). Interesting.

It may just be the widening of the poll sample, since the early polls may have only received actual responses from True Believers. More recently perhaps more people have opinions and/or are answering robocalls. Sheer speculation on my part.