This case is an exception. Twitter drew a line in the sand. It is in exactly the right place.
The PoTUS is threatening to shut down elections in November: he seems to be doing everything in his power to have a national emergency then when people can't vote, to shut down post offices, and to ban voting by mail. Any other problems with the PoTUS, we should address in the ballot box and through citizen activism (not through corporate activism). But when the PoTUS tries to shut down the ballot box or shut down citizen activism, that's different.
I don't think he's likely to be successful, but I didn't think coronavirus would hit us this hard either. In January, it was a manageable billion-dollar problem. We did nothing. Now, it's a multi-trillion dollar problem. Right now, Trump trying to cancel the election is a manageable problem too; by his personality, if he doesn't get traction, we're done. He'll move on. But if he does get traction, we'll have a completely different scale of problem on our hands.