yes but siege up times let blink stalkers walk all over them, they cant attack structures and they are needed vs air.
pure liberator will get wrecked by stalker air combo, because its either kill the air and die to the ground, split siege and lose critical mass vs air and die to both, siege vs ground and have them run away and die to air.
Liberators are one of the last units Terran should build for anti-air; because they are fairly bad at it until they reach ridiculously large counts.
In practice, the kind of mass air army you are talking about is ridiculously expensive, and slow to build up and transition into; such that in almost any game where a player is allowed to pull it off he/she could (or already did) easily crush the opponent long before that point.
Liberators should fight ground units, fighting air is what your thors are for. I only use liberators as anti-air if either
a) I have already killed his ground units or
b) Protoss goes for a pure air army in the late game.
Basically I find that liberator/thor/hellbat/ghost is good against most Protoss army compositions. If he goes pure air I sometimes add a few ravens for anti-armor missile and use my liberator splash damage to help my thors fight the air army. But typically I try to end the game before it goes to that stage, fighting against Protoss in the late game is never a good idea.