So, you think his having the largest poll lead over Trump now is temporary or illusory?
I think it's illusory. I think polls are suffering from many of the same issues that caused them to be so far off during 2016 (Say one thing and do another in the isolation of the voting booth). That plus I think they are likely suffering from confirmation bias.
The poll given to a Sanders/Buttigieg/Harris/Warren/<Insert Dem Pres Candidate here> supporter when presented like:
"Who would you vote for Trump or Biden" is obviously going to be Biden at this point in time, but I can't help but believe that the emotional nature of most Democratic voters will leave some of these poll respondents home on Election Day. When given the option between sanity and insanity, while their candidate still has hope at this point in time, they will chose what they believe to be the saner choice while in the back of their mind knowing it isn't a valid choice because <Insert Dem Pres Candidate here> will be the actual option.
I wonder how the lead would look if the question was more along the lines of:
"Who would you vote for Trump or Biden, and would you still go vote for sure even knowing that your real favorite candidate lost the primary?"