A lot of game designers don’t open their code. The developer of VVVVVV did and the results are interesting. According to a blog post they had 400 pull requests, a port to Dreamcast, Haiku and Webassembly. And localisation is going on. Conclusion: “Only good things have happened. This whole thing has been a really positive experience, and I’m really glad I did it.”
I guess that @RonGilbert had made also only positive experiences?
(Please note that the source code of VVVVVV is available but not under a “free” license.)