Personally, I wouldn't bet on anything but fusion (when/if it is perfected.)
I have read studies that say we have enough nuclear fuel for the next 2000 years (or at least 300-500, if demands increase faster than predicted).
Then again, there are studies that say we could be out of easily accessible ore by the next century.
Nuclear power is not sustainable
Nuclear power is sustainable
Is it a good idea to switch over to 100% nuclear, if there is any doubt that it will last beyond 2100s?
Asteroid mining could potentially solve this (IMO), or maybe just skip nuclear and invest in fusion more.
Anyway, my $0.02