I agree. Given the tiny amount they burn and the little population of Mistings that is present on the planet, mistborn a bring basically extinct, it is very unlikely that they are going to run out of metals due to its acting as a Allomantic fuel.
Feruchemy anyway is such a magic system that it does not consume metal at all. And now with knowledge so widespread about it, I think Hemalurgy if anyone uses, will be used to acquire Feruchemy skills more than Allomantic ones and therefore even Hemalurgically created magic users are not going to be huge in numbers and they will not be burning metals much.