Bitcoin is volatile, but since it is a retirement fund, it is a very long term investment depending on the age of the individual
i would say if you are in your 20s or 30s and you hold bitcoin untill your 50s 60s can turnout great for you, not so much if you are already too old.
and bitcoin does so well in the long run, neither is it going to go to zero.
bitcoin is not going to zero, that's for sure. but it will be a mistake not to diversify your retirement funds. after all, past bitcoin performance is not indicative of future results.
you should hope for the best, but plan for the worst.