how are they going to "go to someone like biden or harris" when we're talking about polling in a general election matchup? those aren't options. the options are sanders or trump (or presumably some abstain/third party option).
what you're suggesting is that there is some substantial number of people who fit the following profile
- support sanders in a hypothetical general election matchup
- do not know that sanders wants to nationalize health insurance
- would no longer support sanders in said hypothetical general election matchup after learning this
do you have literally any evidence for this position? because your polls on health insurance (which, just for the record, have support levels that vary dramatically based on the phrasing of the question) don't really do anything to address the actual point being made, which is that sanders performs well in current general election polling.