@Steve R. - I believe it is a rather technical question as to whether Joe's "ouster" was a coup or not. First, he's still president. However, after next January, he won't be. So as far as the country goes, there was no coup. More likely, it was the democratic chickens fleeing the coop that led to a bunch of dumb clucks like you and me trying to figure out what happened.
As to the legality of a late substitution, it has to do (among other things) with state laws on voting for the next president. Since the presidency is determined by the electoral college, we have to look to the U.S. Constitution, Article II, Section 1, paragraph 2 - which starts with the clause: "Each State shall appoint, in such Manner as the Legislature thereof may direct, a Number of Electors". Which of course means that we have the possibility of 50 different sets of rules from 50 different state legislatures on determining electors.
There is also the fine point that the Democratic National Convention has not yet convened, so the party officially has no candidate. The only sticky situation would be for states that have already nominated their electors AND their state rules require them to support a candidate by personal name rather than by party. However, even that is normally handled by saying "electors are committed for only the first x rounds of voting." The whole point of "electors" is that they are surrogates for the people but have to make a choice for the president. They cannot allow a perpetual impasse to delay filling the office.