Thorium is not a fissile isotope - meaning it can't be used for fission directly. It needs to be transmuted by neutrons into uranium 233 (which is fissile). By this measure, thorium isn't any better than Uranium 238 - which is 99.3% of naturally occurring uranium. So "All but a trace of the world's thorium exists as the useful isotope" - can be applied to uranium also. Uranium 238 is also a fertile isotope - and doing a completely fair comparison, uranium doesn't need to be enriched either... Except the only way we have to convert fertile isotopes to fissile isotopes is to expose them to a sustained, high neutron flux which is typically only economically achievable using enriched uranium (via the naturally occuring U235 isotope). Thorium breeder reactors need enriched uranium as much as uranium breeder reactors. So while thorium has some advantages, I wouldn't say that natural abundance or supply are particularly significant.

[Edit] corrected use of "fissile" -> "fertile"

This estimate doesn't seem to account for enrichment. For every KG of uranium mined, you get a much lower ratio of enriched fuel.

> rough estimate shows that if all our current electric energy demand would be satisfied with Uranium, the reserves would run out in roughly 10 years.

While strictly correct, this is extremely misleading. Current resources would be exhausted in a decade, but resources are defined as the known deposits extractable under current market prices. Should we actually start using a lot of uranium, the price would spike, which would lead to a lot more deposits becoming economically available. This has almost no effect on the cost of nuclear power, as the cost of the raw uranium isn't a large part of the cost of producing power.

The end game there is when the price rises sufficiently for extraction from seawater becoming profitable. The world's seas have ~1000 times more uranium than conventional ground-based resources.

Nuclear fuels will not run out in this millennium.

That estimate for uranium assumes conventional reactors, which fission U-235. That's 0.7% of natural uranium.

For a real comparison, you should look at fast reactors, which use the rest of the uranium. That takes your estimate up to about a thousand years. But the estimate is looking at economically recoverable reserves, and if the same ore produces a hundred times as much energy, a lot more becomes economically recoverable.

Thorium would be 3-4 times more abundant than that. (But if seawater extraction works out, uranium will have the advantage again.)

If it's 3-4 times bigger than the Uranium, and Uranium will last us 10 more years - Thorium would last us 30-40 years. What to do after that?

If there ever appears a new intelligent civilization millions of years after humanity, they'll be very disappointed to be on an Earth without any Uranium or Thorium!