Neither was Harris. Biden endorsed Harris to be the party's nominee, the party delegates agreed and supported her, just as they would have supported Biden had he stayed in the race. All according to established party rules.
When Spiro Agnew resigned rather than be indicted, Nixon appointed Ford to be his VP, and Ford went on to be President when Nixon resigned. The main difference between Ford's circumstances and Harris' was that the voters didn't get to choose Ford as President. Harris did face the voters.
And, in both cases, democracy survives. No threats.
Sure we do: Joe Biden.
If you're unclear on how parties decide their nominees, perhaps this will help:
Understand how presidential and vice presidential nominees are selected at national political conventions. Learn about contested conventions and brokered conventions.
www.usa.gov
Basically, individual states decide who the delegates to the national convention are, and those delegates vote for the party's nominee based on the results of that state's primary or caucus. If this doesn't result in a clear nominee, then there are other rounds of voting, and other delegates, not specifically pledged to the winner of that state's primary or caucus can vote, and the results may differ. I don't recall that happening recently, though.
In this past election season, the delegates pledged to Biden because he won the various state primaries voted for Harris based on Biden's endorsement of her.
But, in any event, these delegates are not some "cabal" who has anything whatsoever to do with the office of the President, or aid in any way with his (or her) decision process. They are chosen by the party on the state level, in each respective state.
So, again, no threat to democracy here.
But, if you find this process unsettling, maybe you should try to work with other citizens to try and create legislation to change the system by which parties nominate their candidates. As it stands now, candidates are not chosen by the nation as a whole, but by the first few rounds of primaries. States that hold primaries later rarely have a say except to choose among those that remain after everyone else has voted. I think a single primary date for every state, so all vote at the same time, would be a better solution, personally.
-- A2SG, though, some people in Iowa and New Hampshire may disagree....who'd pay attention to them then?