Take LoL

I agree, I'm just saying it's an unrealistic stance to have unless you want to play no games at all.

There already is private servers of closed source MMOs

Yes, most done by reimplementing the protocol. They are separate projects that network with the same client project.

Not sure I understand it

Many MMOs have secrets that are undiscovered for years. Many communities spend lots of effort testing mechanics trying to discover how something works, or working together trying to solve a new quest, or piece of content nobody figured out yet. They create wikis trying to catalogue what the player base figured out about the game. When you put it out in the open, all of that is gone, the game is "pre-solved".

I don’t know where people suddenly decided cheating is bad.

Millennia ago, I assume, when competitive sports became a thing.

Getting into hacked lobbies and playing scrapped content was so much fun in MW2. Or hacked zombies in W@W

Sure. Never played that but this sounds like mods and has nothing to do with cheating.

anti cheat doesn’t stop cheating.

And door locks don't stop burglary, but they sure reduce it.