There's always a new 9 year old, 11 year old etc. and always a new gamer of any age, who hasn't ever bought the game. Same with No Man's Sky and Minecraft.
You'd likely make way more if you separate games and DLC, but some companies seem to be fine sticking with just the one. You will still make money as you keep adding things to your evergreen game
And I bought it last year! Just because it's been out for a while doesn't mean people aren't still discovering it. Especially true for a 2d, stylized game like Terraria where maxing out the performance and graphics isn't essential to make it competitive with newer games.
IIRC there's rumors they're working on terraria 2 (or just a different new game entirely) due to the fact that they are hiring some employees that wouldn't be very useful for terraria 1. Also I believe there has been some twitter hints or something.