There could still be a tail event big enough to exhaust even a higher level of equity in a G-SIB, therefore requiring resolution. And we might be uncomfortable about how quickly equity might be burned through in such an event, and we do want to be sure we can resolve a G-SIB. So some GLAC is helpful in that situation.
Unless we have draconian capital standards for equity, some requirement for bail-in debt in addition to equity may well be sensible for G-SIBs. But overall it is appropriate, in my judgement, to allow equity capital in excess of regulatory minima to be counted towards GLAC requirements.