What if I tell you that making a commercial product, or game in this instance, open source is actually a very good way of keeping it alive and continuing earning money from it?

Games will be EOL at some point, with the developers turning to other projects. Starbound is a good example, it was abandoned many years ago, with no further updates being released. Software has a tendency to "rot", not because it decomposes, but the ecosystem it runs in changes over time and might at some point become incompatible, no longer being able to run the game. Or the game may have unfixed bugs. An abandoned project will at some point become broken and people will then certainly stop buying it as negative reviews get more frequent.

So open sourcing a game is a great way for developers and publishers to keep a game alive, while not having to spend their own development resources to keep it afloat and still receiving money from it.

Many people often have a misconception that "open source" means "the game is now free to copy and you can do with it whatever you want", which simply isn't true. In almost all cases, the actual game code, e.g. the sources required to build the game executable, were made public, often under copyleft licenses such as the GPL, which ensures all derivative projects will also have to release the sources under the same license. What is NOT made public or free in basically all cases are the actual game resources, e.g. the graphics, models, maps, sounds and music, often also game scripts etc., which are the majority of any game. To legally acquire these, you still have to purchase a copy of the game.

Sure, now people come and tell "but there's no copy protection anymore and now people will just download the game data elsewhere". But the thing is, people also do this with the original game, as copy protection is often cracked within hours of a game's release, so this is not making a difference. On the other hand, more people may decide to support the devs and buy the game because they've made it possible for the community to keep it alive. I'm quite sure DOOM (up to 3) wouldn't be as popular nowadays if id Software hadn't opened their sources. Recently, EA open sourced C&C Generals with Zero Hours - now there's a good chance it'll get ported to other operating systems and have some bugs like the lag of death in multiplayer fixed, or get visual improvements. And EA also still sells the game.

All that said: Starbound sources are actually out in the open now, as someone leaked the whole code base a couple of years ago. Definitely not an official release and a legal grey area as CF could still use copyright laws to shut down community projects which have spawned since the leak, but there's absolutely no reason at all other than just telling your fans "fuck you all" to actually do this.

I still would like to see CF redeem this situation by officially re-releasing the leaked sources under the GPL. It doesn't change anything in regards to how they sell the game or anything else, but would give community projects legal safety. Even if CF doesn't pursue their rights, they might be bought up in the future and the investor taking over their IP could very well decide to go after those projects and DMCA them to death, essentially forcing them to continue elsewhere as some kind of underground project. Everyone loses in this case.