I know most of the Bethesda RPGs have massive mod support, and there's games like Minecraft that have more mods than anyone can imagine. I would consider those games pretty playable in their vanilla states. Would you say there are any games that were "saved" by modding? Or that are still kept alive by thriving modding communities? What are some of your favorite mods?
Mount and Blade Warband! Base game is just ok, but mods like Diplomacy make it way better.
Then you look at Prophecy of Pendor or Perisno and you've got basically an entirely new game with the same engine and it's awesome. I have hundreds of hours of Mount and Blade, and probably only like 25 of them are vanilla.