If your game depends on an online component, and the online component disappears, yeah, you won't be able to run the game unless that missing piece can be recreated somehow (as Flashpoint is doing).

This is exactly the problem with making games rely on external servers in order to start, as Stadia does (for entirely different reasons).

AFAIK Flashpoint (or some other Flash preservation project) use an embedded server with an embedder browser to make these games work. Most of them were single player or relied on simple common (among game hosters) APIs that are easy to replicate.