Ever the voice of reason.No, not a coup. They didn't storm the White House, kick out Biden and install Harris as president instead. If they had, that would be a coup. Even forcing Biden to resign so Harris becoming president now might be described as a coup. They didn't do either.
Biden remains the president and will be the president until January 20, when his term naturally ends and the candidate who got the most electoral votes in the general election takes over. That's our system functioning as it was designed to do. What we are seeing is internal party politics, and throughout our history we've seen several instances of parties struggling, going through chaos, or imploding. Parties are not included in the Constitution nor are their internal operations governed by federal law. So for all the drama, this isn't a legal problem. It's a party matter. The party officially picks its candidate at the party convention, which hasn't been held yet. So following the party rules, they'll hold the convention in a few weeks and the delegates will decide who will be the party's candidate. That's how it's worked for decades. That's nothing new. That's not a coup.
I don't see why Trump Republicans shouldn't care about this because their candidate is still in the race and to hear them tell it, they believe Trump would crush any opponent. So for them the side show on the Democratic side shouldn't matter. In fact, they should be delighted by the chaos on he other side. IMO it's not obvious that Harris would do better in the general election than Biden. There are other Democrats who I think would match up better against Trump. It'd only be a problem for the Trump Republicans if, despite what they say, Trump isn't a sure thing and they think Harris would be harder to beat than Biden. If Trump were the obvious best candidate that wouldn't be a worry, right?
Each party gets to select its candidate as it rules allow. For most of our history, party bossess selected the candidates; the primary system as we know is a product of the late 1960s and early 1970s. Before that, the party conventions were much more meaningful than they are today. So it's not like the party system we have today is deeply entrenched in our history back to the founding fathers. Some of the founders didn't want to see a party system arise at all as they distained the British party system. If the party doesn't want Biden on its ticket, that's its choice. If the party members are unhappy about it, they can change the leadership of the party and/or the party rules to make the party work as they want. Though from what I've seen, few Democrats are terribly upset that Biden dropped out. Their fight is more about who should replace him on the ticket; there are some who would like some other candidate there instead of Harris.
The voters will still make the ultimate choice of who takes the presidency in November. Thats the contest that most matters.