Wasn't the Stop Killing Games movement about allowing people to keep playing online only games (like MMOs) after the company that made them stopped supporting them?

From Wikipedia:

Stop Killing Games does not demand for games to be supported and updated by their developers forever, rather arguing that they should be required to be left in a reasonably functional state after discontinuation, such as by introducing an offline mode or the ability to host private servers.[10] Scott also clarified that the movement does not focus on the delisting of games from storefronts as long as they are still playable by buyers after discontinuation, and that they do not focus on any financial censorship issues.[16]

So, I don't think people are fine with just renting MMOs? But, I never got that deep into it since I'm not in the EU, and I might also be misunderstanding your question.

EverQuest is still going. So is Ultima Online. My wife's EQ account from launch is still there with all her characters and goodies. As for the others, people were upset when those were shut down. People definitely weren't "totally fine" with that.

There's another aspect though, in that with MMOs you are paying monthly for a service, and an actual quantifiable one. The servers that run those MMOs (current ones anyway) aren't light pieces of computer equipment that you could easily run in your home. It's like going to a bar with a cover charge: you could talk to people at home or drink at home, but you're paying for doing that at a place with some neat decorations and a lot more people than you're likely to meet at your home.

Expanding that metaphor, singleplayer games are you drinking at home. People do not want to give the liquor store the right to come into your liquor cabinet and take back the booze if they decide you no longer should have it.

I don’t think that’s okay, and there are a lot of people that feel the same way. Stuff like that is why the Stop Killing Games movement exists. There should be some kind of consumer protection so that even if the company ceases to exist, players should be able to keep it going in some capacity. In this case likely through private or community servers.

Of course all of these games ought to still be preserved somehow, and I don't think they're saying it's somehow a good thing for them to be shut down.

But you asked why some people are okay with it, and they gave you a valid answer to your question. There is a distinction to be drawn between games that had a legitimate reason to be always online versus games that don't, and that distinction is the reason why some people are okay with it.

For MMO players, most of them understand that these are always going to be ephemeral experiences, this is inherent to the genre. It's even baked into the way that many of them rely on subscription fees rather than a purchase.

The real tragedy with MMOs is that even if servers were to stay up, the playerbase may not, so the game will never be the same if the MMO simply drops to MO. Usually by the time the plug gets pulled, it's because the game had already faded away from what it once was.