source available can allow a lot of things including modification of the source code (and in particular adding quality of life improvements and updating the code to run on modern platforms). Some restrictions like not allowing selling or even not allowing competition (for example allowing the game engine to run only the original game , or disallowing the removal of monetization).

If you look at openage (age of empires 2 reimplementation) the game is not playable 25 years after release and that game is considered a classic, we could lose a lot of very good games or software.