Halo 2 PC is going to be dead entirely in a month. People are no longer going to have access to downloading their games or playing online. Microsoft is preparing something to deal with that wave of negative PR.
Unless I've missed something, there's no end date just yet. BSAngel said an update would be forthcoming in "early 2014" but as far as I'm aware there's been no word since.
Games For Windows Live shutting down will affect several games, this isn't just a single game server that is shutting down, it is an authentication service for multiple games as well.
I am having a hard time figuring out how a re-release of a game that is already on PC would cannibalise the sales of an actual remake on a different platform.