I would argue there kind of is such an expectation. Unless obviously stated in the game description before payment I would expect it to function similar to other games I purchased. And I can still play the offline games from early 2000s without any issues.

I'm saying that because if an online game would be sold with a banner like "we plan to support this game for next 3 years but it might shutdown any time after that" this would absolutely affect my decision to purchase it or to purchase microtransactions inside the game.

If I may be frank, most history isn't worth remembering. There will always be a dozen fans of some niche MMO that lasted a year in the 00's, but people just move on eventually. It's entertainment at the end of the day.