In my opinion, there's two main things that keep the game up and going
The developers support, and the community, the developers announces some updates, and the community get inspirated again and they create some stuff until it's released (TF2 for example), and when the update is released the community have fun with the new additions, they buy and gift the game more, and the developers only get positive results until it cooldowns so after a few weeks another big or major update can be announced and the cycle repeats.
I appreciate sales, but that alone is not enough, we just need that boost of inspiration again just like what happened with the events in V9, The brick physics in V11 and the shaders in V21, it doesn't have to be that big, but any new addition would be nice.