The player isn't sealing off anything of the "GM's game world". The "GM's game world" and the "shared game world" are not the same concept. Limiting it to just what the GM control's with the term "GM's game world" means everything in the world except the characters. The players control their characters.
This idea that everything belongs to the GM may be the root of your misunderstanding.
Again, answer the flip - why does the GM get to carry an "undisclosed expectation" that they can take the player's concept and use it after the player leaves without permission.
Since the beginning of RPGs it has been established players characters are not part of what the GM controls, so that's the far mroe reasonable question.
Logical fallacy, Reductio ad absurdum. That is not what is being suggested, just that the character belonging to the player is written out of the story. Everything else still exists.
Again, PCs are not part of the "GM's setting". They are part of the shared story told by everyone around the table, which is not solely owned by the GM.
Also, after a 17 page thread on it, declaring that it is an "extreme view" is just rhetoric. Look at all the discussion and realize this is not extreme, and is common in a bunch of games. Please don't mischaracterize it as such.
This isn't a point under discussion, not sure why you bring it up. Of course you can refer to prior events that happened while the character was there. It's about turning them into an NPC without permission and running them in future sessions. Please don't try to muddy the issue, or bring up unrelated points trying to make an argument.
Everything else you wrote was a continuation under the mistaken opinion that people are arguing against the history of the game that was played, which no one was. I'm leaving it out as it's not a point under discussion, everyone is fine with the character having had an effect on the world when played. This topic is about the DM deciding to take control of a PC without permission.