It was perfectly legal and with near perfect consensus, including with Joe Biden’s endorsement and support.  The two parties have always  finalized their nominees at their conventions. 

 Exactly what I thought. I had the feeling that this was only an inflammatory note in a Twitter style but it's better to ask. 

 Yes, the Constitution only speaks to general elections.  Parties choose their nominees according to their own rules, whether Trumpers like it or not. 

 In short: no, it is not illegal.

The OP note is a little incomplete. Biden earned the vast majority of the delegate votes from the primaries (i.e., the public).  But, the subsequent delegate vote didn't actually take place until the convention - after Biden dropped out and endorsed Harris as his replacement.  So, unsurprisingly, instead of casting those delegate votes that were earned via the primaries for Biden, they were cast for Harris.

I think part of the point of the OP is that the public has effectively been disenfranchised from their votes in the primaries; and, I agree that's a shame.  It's not an ideal situation that Biden and his supporters (including Harris) left the party with.  But, as long as the party follows its own rules for resolving it, legally, that's all that matters.  FWIW, some other parties don't even participate in the primary process for input from the public when choosing their nominee... they just have delegates vote.