A poorly failed and failed coup attempt has an obvious historical antecedent, for one thing, in https://en.m.wikipedia.org/wiki/Beer_Hall_Putsch

Second, the fact that they tried at all, even if it was stupid, is concerning and worth taking seriously.

Also, all of the laws about who's president and who's in charge are "made up*. You understand that, right? If enough people start to disagree on fundamentals like "accept the election results" you're going to have a failed state, civil war, that kind of unpleasantness.

They probably assumed a few key players would have folded and they would have gotten their way, or trump would have declared martial law or similar. A lot of these people seem to want a dictatorship. Some of them seem to want to dismantle the state.

There's no referee. There's no computer enforcing the rules. If a bunch of people with guns say "no. This guy is in charge now. We don't care about the election" that's a crisis that needs to be taken seriously. If it was four guys outside a Walmart in Phoenix, fine, only a little concerning. This was, what, tens of thousands of people, who traveled far to get to DC, with the support of a large chunk of one of our two major parties. That's serious. That's 1923 serious.

To be like "oh that can't happen" is just head in the sand. Recency bias. Wishful thinking.

the idea of even recognizing that as somehow a legitimate political process is a joke.

You're looking at it all wrong. Legitimacy isn't magic. There's no ref. It doesn't need to be "legitimate" it just needs enough might for people to accept it. And coups aren't legitimate to begin with!