I vividly remember a downloaded game telling me they had run out of available licences, once. Can't remember exactly, but I'm pretty certain it was on Steam. How you run out of numbers still rascals me, all these years later. And I say this as a software dev.

Want to sell: Microsoft flight simulator license DM me to discuss price.

Maybe there's a middle ground, where instead of just letting a flood of people all download your game on day one, the publisher like pre-downloads it onto some sort of physical media, and then sell copies of that physical media. That way people could get into the game immediately when they receive their copy without having to wait on the same 6 hour download that a million other people are also waiting on, that download activity doesn't interfere with the bandwidth of people trying to play the game, and the physical availability puts a sort of temporary artificial limit on how many people can play at once.

Any company worth any money does stress tests of their servers to simulate different scenarios with different loads.

Either they were overconfident, or simply didn’t think QA testing was necessary or worth the cost.