Tittle defenses is a meaningful metric but obviosly not the only one first of all because it assumes that UFC=MMA and we are not asking for the GOAT UFC fighter but about the GOAT MMA fighter.
Just as example, why Rockhold's win over Jacare, or even Machida; or Henderson's wins over Bustamante or Misaki, are less meaningful wins than Franklin's wins over Quarry or Loiseau? Thats what your criteria would set and I definitely dont agree.
Anyways, I agree that Silva >> Adesanya at this point. Not just because of adding nº of tittle defenses vs the likes of Coté, Leites or Lutter, but also because of the decisive manner in which he dispatched his competition and, from a p4p standpoint, because of his top5 wins in three different weight classes.