Isra said:
These games inherently sacrifice dynamism and so many other things. You can't cut off the quest giver's head or find an item of great power at a low level. You can't steal the merchant's entire inventory or have quests which truly impact the game. In many ways your experience is restricted so as to not disturb the experience of others playing around you. In many ways your entire existence in the game feels like a shoe in. .
You bring up a lot of good points, but I'd like to address this one in particular.
I feel that this is the same in a lot of ordinary games. You're not allowed to really affect the world, like it's the developer's toys and they're afraid you'll break it. The thing is that complete freedom might not be all that desirable as it would most likely just end up being a wasteland. If anyone could kill off the quest-giver then there would have to be either a respawn (like there is in WoW for example) or the quest itself would probably be lost forever, and then what's the point?
There are a lot of practical aspects that one has to consider. Is every game supposed to play like Rust then?
It really irks me that I'm not able to effectively impact the game-world in a lot of games (like how I can even break chairs in Skyrim), but in the case of MMOs I'm not sure if they would be well served by having everything killable or breakable.