I do not think developers should be compelled to add single player modes to multiplayer games, but I do think that server software should be open sourced as a multiplayer game is shuttered. I believe the studio is entitled to royalties if a server host is profiting from their discontinued game, but I also think that servers for discontinued games should be allowed to be run as nonprofit, charging only as much as necessary to keep the server active and healthy, and not be charged royalties.
I say this having not watched any discussion on the topic, nor knowing anything about how any of this works.