A viable business model isn't really a mystery and, on a related note, KSP 2 was never a necessity.
Maxis has thrived for years, for example, on expansion packs for the Sims franchise while delivering ridiculous amounts of content, gradually enhancing feature sets on top of a pretty solid base game, and making absolute bank in the process. In my opinion, this is currently the most elegant solution for all simulators and would've made for a premium pipeline to deliver Colonies, additional solar systems, texture & technology packs, multiplayer features and more. Why TT ever tried going the route they did boggles my mind.