Earlier quoted context omitted.
It is not. Most of the full online game stacks are not open source and most of the open source ones are poor and under featured
That could change once a law comes out requiring all new developments to be designed differently. Besides, no one is talking about open sourcing the server code. Releasing binaries and patching the client to talk to a local instance is perfectly acceptable. A developer would then just need the ability to redistribute compiled builds.
It might make a community rewrite of the server code easier, but that would likely only be attempted for very few games.