If I cast "Greater Restoration" and say I'm going to cure your of your Warlock pact, and roll a nat 20, and you are vehemently against this, it's not "for the sake of fun" if I go ahead and "cure" you of your chosen character traits and path.

I hear you, and I'm not rigid in spell use within reason, but this is well outside just RAW and more into the latter part of everyone having fun at the table. Your fun shouldn't come at the expense of another player having to give up agency over their character, which are personal avatars people can sometimes be quite attached to.

If you're in a table where characters are dying left and right maybe they aren't. But even then, if they don't want it, that's the red line. Just like using Mind Control on a party member to do something unspeakable. RAW could they? Sure. But unless this is a game built on betrayals or where players are expecting a PvP element, absolutely not. Because the IRL consequences of this and the real anger a player at the table may feel trump mechanics.

Everything in moderation, everything on balance. Player agency is something you should try not to let other players trample on. And even as a DM, it should be subtle or not at all when you are moving the scenery to guide something. Again, subjective.