That’s true, Witchers are not born, they’re made…though the age at which children are brought to the Witchers is frequently quite young. Hmm.
I don’t see why a game with Witchers and non-Witchers needs to make Witchers more important. Especially since BITs are the main driving force of Burning Wheel, not combat prowess. I think it’s entirely plausible for a tangle of plots to involve monster-hunting as well as political intrigue and other aspects which appeal to different characters. Likewise I think it’s plausible that while Witchers are assuredly better than average soldiers, characters exist who can give a Witcher a run for their money in swordplay.