The great thing about FRP is that the material is directional.
That means you can align the fibres along the expected stresses in the frame, and use less material than would be required with a material that's equally strong in all directions, but weaker than the maximum strength of the composite material.
As long as you get it stiff enough, that is.

Aircraft have been built using composites for more than half a century (longer if you count wood), and they typically stay in the sky. Rarely does an aircraft crash due to an FRP material failure.

But I wonder if we can assume ultra-light-weight bikes to be built to have a long frame life. It would seem to me that, in the quest for ever lighter bikes, they would turn to FRP, not to match the product life of the lightest metal frames, but to be able to cut down even further on product life, while still being able to build a bike that's rideable and doesn't break right away.
People who buy CFRP bikes for many thousands of dollars probably have enough money to be expected to buy a new frame every few years, and throw away the old one. That makes sense to me, at least, since ridiculously low weight seems to be the goal for some, no matter what the cost.
But this reasoning is only regarding the really extreme bikes, not the ones "normal" people buy.