The decline has already begun. Multiplayer is barely functional and downright boring, and Singleplayer, while a good foundation, is barebones, buggy and riddled with questionable design choices. If it wasn't for the modding community fixing many of the small problems with the game(especially character progression), I couldn't even play the game at all. Once the novelty wears off, singleplayer will slowly die too, and that will most likely be within the next 3 months. The decline in playerbase has already begun, which to be fair is to be expected, that's how all games go.
The issue isn't that there's bugs and problems, Warband's beta had it's fair share and it continued to grow. It's that they never get fixed. Bannerlord's multiplayer has basically died, and that's an issue considering the artificially inflated gaming numbers due to the epidemic at the moment. I'm not sure if you were here during the beta, but ~90% of the problems and balance issues brought forward by the community, which had a general consensus that they should be fixed/balanced, did not get resolved in 9 months. And what's worse is that whole sections of the combat system have so many issues that we don't know what are intentional features or bugs half the time, and to top it all off TaleWorlds won't communicate. Nobody expects any community to "flourish in a unfinished game", only that it seems to be stuck in near permanent developmental deadlock and we have zero idea when or even if issues will be resolved.