Isn’t nuclear a fossil fuel as well? While not quite the same thing, I don’t think we can manufacture uranium (or whatever the reactor uses to run), more than once?
These predictions also generally include a healthy amount of assuming growth of energy in the future, too, not just projecting today's use indefinitely (although not necessarily predicting exponential growth either).
This also ignores what I would consider fairly good odds of non-terrestrial sources we may find on that time frame. Plus I'm still holding out hope that one of our fusion approaches will eventually pay off. (Dyson spheres are nice & all but there's still something to be said for being able to generate power in concentrated manners where you want it, rather than having to collect it at scale from an uncontrolled reactor.)
[1] For the fashionably misanthropic among us who may feel that statement may contain too much optimism, "total civilization collapse" would be such a "critical change".
In the longer term (hundreds of years), humanity should be worried exhausting non-renewable resources, but in a shorter perspective the climate change is a bigger concern.
If we can efficiently extract uranium from saltwater, that value jumps to 60,000 years.
If instead we switch all reactors over to breeder types instead of the current light-water reactors, our 200-400 year estimate then becomes more like 30,000 years