So the attempt by philosophers to escape the age-old raison d'etre of philosophy, i.e. explaining why all people must accept this belief (concerning 'metaphysics, ethics, or aesthetics') and not any other, led to mysticism--or at least was initiated by people with an orientation towards mysticism.

I would say that the process of philosophy--arguing about extra-rational knowledge--made language inadequate, not that an inadequacy of language failed philosophy.