Ah yes, closed source, such a dealbreaker, as if 99% of the other games weren’t.
Don’t get me wrong, I have nothing against open source games, it’s just not a viable monetization strategy for most projects, and people gotta eat. There’s reason why most open source games are either passion projects or old games that have been open sourced simply as an act of kindness towards the community since they generate pretty much no revenue.
I could see that being a thing, but the line between the engine and the game itself is a bit blurry in this context. Copyrighting just the assets and content would often not be enough. There will always be a good chunk of game code which isn’t strictly part of the engine but under this model should remain closed source, otherwise people could just bring their own assets.
Frankly I’d be satisfied with companies open sourcing their games after they stop supporting and/or selling them, mostly for preservation and all that. I think that would be a great middle-ground.