I can't give John Carmack and ID Software enough credit for releasing the source code of their games to the community as they moved on to bigger and better things. I can easily play Doom on my brand new ARM based Mac with extra features that make the original game even better with no need to screw around with emulation or virtualization or any other bubblegum shoestring hackjobs to get it working. They provided the entire industry with the solution in 1997. When you have deemed a product as end of life and not worth putting any more time or money into, then release the source code for the client (and the server if there is one) and allow the community to take on the challenge of making it better and keeping it running. If you do that, you can even keep selling the game as an open source engine alone isn't enough to run the game. You still need all of the assets. That's why you still have to buy Doom or Quake even though open source projects like GZDoom and Quakespasm are free to download.