They aren't being nice, they're doing something more cost effective than keeping a server up for another year. They wouldn't do this if it actually cost them money, and being nice means going out of your way, they just don't want loyal fans using bad word of mouth and hurt their image.
They'll shut down the halo 3 servers in a few years to force people to upgrade to the new system and it'll just make people angry again.
This is why a multiplayer is not an excuse for a short or bad single player experience, because game companies are cutting players off from servers sooner and sooner lately in order to 'encourage' them to buy the new product, and people who like to keep and save their systems to go back to won't have a very interesting library.