I work on Gang Garrison 2, a game still actively developed which is also reliant on a dead game creation tool (Game Maker 8.0), so this is sadly familiar.
Unity is closed-source; I'll bet in ten years we'll see the same thing happening again, with people trying desperately to find some way to run these old games that were developed on the no-longer-supported Unity platform.
Dunno, I'd say Unity has enough traction behind it to not suddenly disappear. It's a major contender now.
Developers write their own code in unity (C# or JS) as well as bringing in their own assets (music, art, etc). So even if the unity engine disappears, authors will be able to release their work in a meaningful fashion.
