That’s assuming a lot of ifs resolve our way, and without power needs increasing. It’s more sustainable than coal/gas/oil for sure, but with current energy development needs it’s barely long term (IIRC about 60-140 years)

Also, on centuries timescale, we will need to find more fissiles in space. And according to our current understanding of the universe, they should be quite rare, especially compared to hydrogen.

Basically, figuring out fusion power would solve our needs for the first level on the Kardashev scale, and has the potential to be portable fuel for the rest of the lifespan of the universe.