Is it really fair to count framerate as a barrier to immersion?
Yes

'I just looked at five cars and the world went into bullet time'
Also, I think camera perspective is less about immersion and more about overall design:
First person pros:
greater accuracy at long ranges, doesn't need a powerful animation engine to point the gun in the right direction, handles indoor and outdoor environments equally well.
First person cons:
Platforming is very hard, melee is extremely complex to pull off correctly, it's hard to tell when you're surrounded, distances are hard to judge.
Third person pros:
melee is much easier for the player, distances are easy to judge, you can see your feet for platforming, you can see hazards near you (grenades, being surrounded, projectiles, etc.)
Third person cons:
Expensive animations or complex procedural blending needed, aiming ranged weapons can be a huge pain unless the camera moves very close to the character (GOW), transitions between indoor and outdoor environments are awkward, unless the ceilings are unnaturally tall (the thing's four foot tall air ducts, GTASA's 15 foot ceilings)
Not an exhaustive list.