It doesn't have to be "endless", that's a ridiculous requirement that not even "renewable" energy can satisfy, since the Sun won't last forever. However, if it lasts long enough, say
up to a billion years or more, then I think that's long enough for the foreseeable future.
Even if a billion years is too optimistic, reduce that by six orders of magnitude and that's still energy to last us a thousand years, not including thorium which is 3 to 4 times as abundant in the Earth's crust as uranium, and which can also be used as a fuel.
Nuclear fission is just as worthy of consideration as renewables with regards to the future of energy production.