Like you said, it probably won’t happen for a while, these games are on GP and it would be silly to get rid of a significant portion of the game whilst its still a part of it.
If TC makes something akin to MCC, I could see TC getting rid of it then. These games have been supported for a while, to their credit, just like the 360 Halo games were, those were supported for a much longer time than Halo 2 was, they only had 6 years on that (both the Xbox and the Vista versions incidentally had the same amount of time). Halo 3 will have been supported for 14 years. A lot of games don’t make 4/5 years.
But I don’t think we are near this avenue yet. Something to think about though. I don’t think TC will do it unless a long time passes or a viable alternative comes along (i.e. a remaster). I could see Gears 1 going offline if I’m completely honest though.