Solar and wind power are not sufficient for spacecraft, and many other applications. Fusion power still has very interesting applications.
If anybody succeeds in working out D-3He fusion, that could work in a spacecraft. (D-T, no.) We could probably scare up enough 3He to use for that, if there weren't too many.
Outside the frost line there is a lot of water and a higher percentage of D relative to H so it seems possible to "live off the land" between the stars without being dependent on starshine. A D-D reactor would produce ³He and T, a lot of those products would burn up in the reactor because the reaction rates are high but it would probably be possible to separate some of those out and use it as a breeder reactor that makes fuel for D-³He and D-T reactors elsewhere. I could picture the big D-D reactor running on a large comet or dwarf planet like Pluto producing D-³He for smaller reactors on spacecraft. (D-T not only produces a lot of neutrons but the T has a half life of 12 or so years and won't last for long journies.)
My guess is that interstellar travelers would develop a lifestyle that works around the frost line, where generic bodies above a certain size have liquid water inside. If they were grabby they might consume Ceres or Pluto but might not really care about dry, idiosyncratic worlds like the Earth and Mars.
Having got used to spending interminable ages out in the infinite chill void, they probably have come to prefer being there, so have no desire to roast deep in a stellar gravity well. Their equipment might not even work if warmed too much.
Fine. How do you power a Europa base with Solar? A Neptune probe?
Moon base can be fine with power beamed from a satellite or plain mirrors in orbit, no atmosphere in the way. Might end up being still cheaper than hauling nuclear reactor there plus all the infra to reliably dump waste heat from it.
I guess you're right that solar is useful for 99% of spacecraft -- in that they use it currently. Not a very useful observation
"and many other applications"
Power in polar regions. How well does Solar work in Antarctica? Or Alaska for that matter?
The main difference is that literally anybody can make it, not just "oil exporting countries" and "fuel refiners". And, will. And export excess production when local tankage is full.
Either that or a place near one of the poles where you get water and lots of sunshine. A small fission reactor is a handy thing to have, however.
> Fine. How do you power a Europa base with Solar?
A lot more solar panels, or wire loops harnessing Jupiter's magnetic field and Europa's momentum, etc. Fission is still cool for that.
> A Neptune probe?
Now we enter the nuclear fission territory. Maybe fusion, some day.
For those interested in near term answer:
https://www.nasa.gov/mission_pages/tdm/fission-surface-power...
Do we know how to store it properly yet? How does solar pan out in case of surge ( eg : very cold winter night )
Anyway unlike fusion, seasonal thermal storage is viable and available now, and will be scaled up in immediate future. Also, with electrical vehicles inducing massive investment into the grid, there will be both pressure and resources to solve the rest.