Reminder Obsidian took just 18 months - from concept to completion - to recycle a game engine, build a whole wasteland in it with recycled assets, add a full coherent Reputation system to it, massively improve the skills and perks system, add traits, and still had time to write a narratively deeper base game and 4 narratively deeper DLCs and with some of the best-written stories and characters in the entire genre, thus creating what is widely regarded as the best entry in the entire series.
The lesson?
RPG players couldnโt give a shit if the engine is brand new and got โWay Twacingโ and all the bell and whistles or not, so long as itโs immersive, maximises choice, and feels faithful to the IP. And that doesnโt take 15 fucking years.