The democratic voters do not feel disenfranchised, or feel what happened was against the contest rules, because it wasn't.
Joe came to his senses, and as he did, sacrificed his own ambition, for the Nation after the interventionists pointing clearly to the lack of confidence after his debate, that he was on shaky ground on confidence of being able to last another 4 years, at his age with his own voters....
And of course, the Biden campaign's own internal polling after the poor showing debate, showed he had no chance of getting all democrats out to vote for him. But he also foresaw chaos at the convention could occur picking a new candidate to replace him....which would or could spit the party and democrats lose anyway, with or without him as a candidate....
When he resigned from running again, by choosing the vice president as his heir and making it very vocal...with just 3 weeks before the convention, Biden set democrats on the path they need in order to beat Donald Trump, or at least have a chance at it, with 1 candidate to support and not division, and all the money from the Biden / Harris campaign in her campaign kitty.
It is still a Hail Mary, no doubt....but it will energize the democratic voters to get out to vote for their other candidates running....their feelings of despair, would have kept them home, and not voting at all.... Which would be doubly worse for the nation without democrats in at least one part of Congress, holding a Democratic majority, to hold back Trump from all of his authoritarian wishes!