We need manganese. We currently get that from open cast mines. Those are pretty terrible, so we need to look at ways of making them not awful, or at alternatives.

Deep sea mining doesn't really do much for either.

Nodules are never going to provide enough to meet demand, so mining nodules doesn't get rid of open cast mining. We don't know what the impact is of mining deep sea nodules, but it's likely to be pretty bad.

If open cast mining had a history of attempting to fix the mines (eg re-wilding) when they'd finished we could trust them a bit more when they want to mine new untouched wilderness. But they mostly don't have that history.

EDIT: for an example of the harm already caused: https://www.nature.com/articles/d41586-019-02242-y