The purpose of a primary is to help the party pick a nominee that has a better chance of winning the general, which is, in turn, the purpose of a political party. The mechanism of binding primaries was set up by party leaders after some bad choices (especially in the 1968 Democratic Convention). This time, the prospective candidates decided that a blitz primary wouldn't serve its purpose. If the voters punish them for this decision, then it will have proved a bad one, but it's neither unprecedented nor undemocratic.

If the presidential candidate wishes to hand the reins over to his vice president, why is he not allowed to do that?

There is no way they could have held a second primary in the 28 days between Biden dropping out and the DNC. Polls said Democrats wanted Biden to drop out, Democrats had already elected Harris once, and polls said Democrats were happy with Harris as the nominee. That's about as democratic as you could get given the situation.

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.