How is that supposed to work when the main product is nostalgia? It's a mod for people who think the first-party expansions aren't true to the core of the original game - how could an independent game with completely new IP ever have the same draw?

You really can't compare this to something like DotA, where the original engine and IP was basically set dressing for the new game built within it. People were primarily interested in the mechanics - which is why DotA-the-game and League of Legends were able to become so popular.

I don't think this is true. I think what you may be thinking of is many hit games did not create their own game engine.