 One intuition might be that Ext suffices for constancy. But a different intuition is that a quantifier meaning the same on all universes in particular should satisfy Isom, which forces Q to be the “same” on all universes of the same cardinality. These two ideas are incompatible, since together they would imply that Ext implies Isom, which is manifestly false. Clearly, the vague notion of meaning the same across different universes admits of different precisifications. On closer inspection, it seems unlikely that there is one precise version that would accommodate all intuitions about sameness.