Honestly? They're probably going to continue to offer the game on Steam and it will probably continue to trickle in revenue. Most people who are interested in the game have already bought it, so it's never going to draw in millions again.

The game is currently listed at $15, but goes on sale frequently for $3. I bet many new players would be happy to pay steam $3 for the cloud save and steam's easy game management, as opposed to compiling from source for every new PC install.

Finally, the gameplay loop between Rogue Legacy and Rogue Legacy 2 are pretty similar. This means the the original is a good extended demo for the sequel which is .$25, but up to 50% off. Frankly, it's a great marketing move at this stage in the game's lifetime.

The assets aren't free and open source. If you want to legally use the open source code to play a game, you'll either need to buy the game to get the assets or use an alternative like freedoom.