he can stay in until it becomes impossible for him to win the nomination. At that point, I expect him to withdraw and throw his full support behind Biden.

There is a lot of emotional investment in running for president. A decision to call it quit takes some time. You surely don't believe that Sanders would maintain the campaign operation for the purpose of paying staff.

Let her spend freely.

Almost every campaign, whether the candidate drops out or goes to the election and wins or loses, ends with debt. So they continue to raise money to retire the debt, so I really can’t agree with the premise of this OP.