How does 'continue buying unethically sourced cobalt' improve the state of things?
Ideally, it is a dual solution of don't buy the unethical kind, and set up an alternate ethical supply chain to the same region so workers can switch to a better work place.
But, if the latter is not possible, one can at least help decrease the market for unethical cobalt by ceasing purchase.
Plus, it is not a matter of seeming morally superior. If one option is morally better than another, it should be chosen, regardless of the consequences.
So, for instance, if Jesus is correct, that it is immoral to be a path of scandal, then one should choose to not be a path of scandal, even if the state of the world otherwise remains unchanged (e.g. unethical cobalt mining continues).