Why is everyone assuming Sanders is finished? He should finish with a gap of 89 delegates to Biden, give or take. Many things can still happen.
I think the debate on the primaries has been contaminated by 538 / other polling sites. According to their models, whoever won the last primary or came ahead in the last round of polling has overwhelming odds of winning the primary. But we already saw these overwhelming odds shift from Biden to Warren to Biden again to Sanders and now back to Biden. How overwhelming can they be if they shift so much?
Maybe Sanders will do brilliantly in the following debates, maybe Biden will do poorly, maybe some new facts will come out that will help one candidate and harm another. Why assume it's over when they're not even that far in terms of delegates?