The point is that Democratic voters didn't get a chance to have their voice heard. Conducting polling is post-hoc rationalization for Harris being installed by party leaders in unprecedentedly anti-democratic action.
Yes I agree that Joe Biden had the legal authority to step down and appoint Kamala Harris as his successor to run for President. No, I don't think doing that is democratic. What would have followed the democratic process would have been recognize what everyone else knew way in advance and stepping down early enough for potential nominees to run to be the candidate.
Criticizing the Democrats for being anti-democratic here would carry a lot more weight if the Republican nominee wasn't responsible for J6 and the fake electors plot.