Look at TF2--it could foreseeably be modified and anyone can run a server for it. It may be possible to preserve it. Blizzard's Overwatch on the other hand, which is somewhat of a successor to it, they won't let the game to work at all without their servers since it's a gigantic, centralized black box. Those servers might go away.
The only reason WoW is preserved for the future at all with third party vanilla servers is because during the beta they made a mistake and leaked a ton of debugging info and world data early on. Ideally (from the company's perspective), we wouldn't even had that and one of the biggest blockbuster MMOs in history would simply be lost.
Not picking on Blizzard in particular, but they've gone from a decentralized model in the Diablo 1, War 2 BNE, and Starcraft 1 days to massively centralized. If there isn't a profit to be had, I think they'll be very happy to let the history die there, which is very unfortunate and seems to be a model many AAA game companies are taking at this point, not just Blizz.