A lot of old games have become unplayable on modern hardware and operating systems. I wrote an article about how making games open source will keep them playable far into the future.
I also discuss how making games open source could be beneficial to developers and companies.
Feedback and constructive criticism are most welcome, and in keeping with the open source spirit, I will give you credit if I make any edits based on your feedback.
Cataclysm: Dark Days Ahead and Dungeon Crawl Stone Soup are two examples off the top of my head. Traditional roguelikes are often open source.