In my opinion, the best way to ensure your game stays relevant for YEARS after its release is to incorporate mod support in any way, whether it be Steam mods, or something like Nexus mods. Both would be preferable, but anything is better than nothing.

How many people would still be playing Skyrim today if there were absolutely no mods allowed like some games?

How many would be playing GTA 5, or even RDR 2 without SOME kind of mods making it more enjoyable or completely different?

Just allow your games to be modded, and support mod development, and your game will go down in history, I am almost positive of.