Most people I know with last-gen systems only keep them for certain games like BO2 and GTA V. And GTA already has been remastered. And of those people who still actively play last-gen, they are almost all upgrading by next year. In fact, this Christmas most people on my list did.
BO2, to me, is an example of how DLC could continue. Most of my Xbox 360 friends still play BO2 to this date, they prefer the MP, and love having Zombies on it. Ghosts was a failure to most people, and AW is a love/hate game, some play it, others think it's too different. Hell, I hopped on a couple of days ago and apparantly BO2 still has active league play seasons.
I just think that Treyarch could easily do it. I'm sure they want to, too. I'd personally like more like 6 map packs per game anyways, but Zombies couldn't really interfere with IW's DLC or game. Extinction is a pretty different mode, at least gameplay wise to me. MP players all get the next game anyways, even though most I know never buy the DLC. If they want DLC for either game, they can get it. Both games have different styles of MP and co-op modes, so it would be down to personal choice. However, I could see how if a DLC included MP that it could interfere things, due to the competitive community, all that MLG shit and whatnot lol.