To add to this, in the gamedev space there are a bunch of middleware libraries that are commonly paid for: fmod/wwise, multiplayer networking sdks, etc.
Thinking about this a bit more, it seems that the reason there isn't a good way to sell licenses to software libraries generally is license enforcement. Unity and Unreal have a licensing system built-in that they enforce against gamedevs. Normal server software has no such thing.
That means the only threat you have as a producer of code (once the code is handed over) is the threat of withdrawing service. That means the only ways to sell licenses are:
* Build your own licensing service (or offer SaaS)
