That's like saying we won't run out of oil because we are producing carbon and oxygen... if it's not in useful form, it's perfectly sensible to say that no new fuel is being produced.

Normal uranium reactors produce a mix of plutonium isotopes, which are hard to separate and end up as nuclear waste. NASA needs pure Pu-238, which has to be made in specialized reactors.

Iirc, all Canadian CANDU reactors produce large amounts of weapons-grade plutonium.

Which is exactly the problem. RTG needs Pu-238, and can't use weapons-grade plutonium, which is mainly Pu-239.

That design has also been altered by India to consume the plutonium by-product, using it as fuel.