No multiplayer is truly free. If Bannerlord would suddenly become "Free2Play", it would mean that Taleworlds are monetizing it in a another way. I guess there would be more skins and possibility to convert real life currency to loot, so they would make something out of it. Okay it doesn't sound like a worst case scenario, but what would happen to multiplayer modding? I don't know any F2P mp game that supports modding and is free. It's like this for a good reason: Why would company allow something like modding when it can bypass all their in game store items and players can acquire them for free?
This would work only if Taleworlds somehow makes this a best game ever (doubt), but in all other cases it would harm the game even more to make it free.