And just where will you get plutonium in industrial quantities without a uranium-based fission reactor to generate the neutrons necessary to make plutonium? Are you forgetting that plutonium does not occur in nature, and that it's purely a synthetic product of nuclear fission (apart from perhaps a few random atoms that occur as decay byproducts in uranium ore)?
I'm not forgetting anything. It seems you didn't read my post very carefully. Here it is spelt out:
1. Reactors are built from naturally occurring (unenriched) Uranium.
2. Plutonium is a byproduct of the fission process in those reactors.
3. Plutonium can by separated from the other reactor products chemically.
4. Plutonium can be used for bombs.
5. So Uranium isotope separation was unnecessary.
6. So all the expense of Uranium isotope separation was unnecessary.
7. However, as I and others have said, this is only clear in hindsight.
There's lots of good articles about this on wikipedia if you need any more info.