Depends on many things.

But ideally you want some game logic on the server side, to prevent cheating.

You can just sync world state between players, but that will always cause weird glitches.

I don't develop games, so I'm no expert, but I my backend would have the 'one true' state of the world, and players would only interact with it by using verified actions.

I did one such game, but it was turn-based tabletop, not very comparable to open-world games.

But those studios could still provide tools necessary to keep the games playable after they no longer want to support it.

No one is asking for laws that force studios to foot the bill for indefinite support. But games don't have to built in such a way that access can be entirely removed at the whim of a dev or publisher.

Lots of MMOs have private servers already. It's not a revolutionary ask.

I think the point is more that when they decide they don’t want to run the servers anymore they should have to release the server code or in some other way allow the server to be run locally so that the game isn’t useless.

Also, until about 15 years ago, it was completely expected that you would buy a software once and they would still run their servers. That’s why windows used to cost like $350 in mid 2000s dollars, you could still reasonably expect to get online updates for it years later.

It's not up to me to pay extra for servers, they should take the costs of them into account when deciding to make an online game.

Games like World of Warcraft make you pay for DLC and a subscription, which is ridiculous. OSRS just asks for a monthly subscription, is that a model I want to see expanded? Absolutely fucking not. On topic though, games with that model should still have to release the entire game for free after they close the official servers.