Just discovered that Mindustry is open-source! GPL3 from the looks of it:

https://store.steampowered.com/app/1127400/Mindustry/

https://github.com/Anuken/Mindustry

Seems to be doing well, so there's some anecdotal evidence open sourcing your game doesn't have to kill your sales

"People will not pay for my game if they can get it for free" is kind of a 90's anxiety.

Nowadays your problem is "Nobody knows that any version of my game exists, paid or free"

As far as I can tell the only real risk from open sourcing your game these days is somebody putting out a fast-follow competitor going after your own audience, but I assume you can make that difficult for them with a GPL or dual license

And it's not like the law ever stopped some folks from fast-following your game anyways, or even directly ripping off copyrighted assets