I don't care a lot about Twitter itself, and I'm willing to entertain the notion that it could function equally "well" with a much smaller fraction of the workforce...

But this is something that should be implemented gradually. Here is someone who just stepped into the company, and he's laying off people en masse. Even if you concede that Musk is some sort of genius (which I don't, but that's besides the point), that doesn't mean that you can just walk into a company and tell from day 1 who is productive and who isn't. And even if you could, making redundancies obsolete isn't something you can just do in one go, even if certain people and processes are inefficient there would probably be a need for a transition period where the institutional knowledge can be transferred.

Given everything else I know about Musk and this deal, this looks to me like a knee-jerk action, not a careful business decision.