In round figures you get max benefit at current Nb prices if you...

In round figures you get max benefit at current Nb prices if you add (say) 0.25% Nb to your iron ore - but with deposits like Luni (West Arunta) sure to be mined the price will come down and more will be added to the steel.
The new light weight corrosion resistant metal will be high strength steel. Until the Nb runs out, and that will be long before the iron ore runs out.

As for REEs - once again price will determine the deposits which are mined, and that then will determine the price.
All with the usual market fluctuations which we investors must use to our advantage.

Further we have only scratched the surface of uses for the critical metals - and I include Indium and Antimony in that category.
One could build a profitable Family Office by investing in the long life deposits which get developed.

I'm here on PGD for more of the 3% - gotta be there somewhere. LOL.