According to a research from last year

Sixty-six percent of the entire planet is deep ocean (≥200 m), and our data show that we have visually observed less than 0.001%, a total area approximately a tenth of the size of Belgium.

If we have seen so little of the deep sea, let alone study it, I am pretty sure scientists cannot really make assessments of the impact of deep-sea mining.

I suppse this is a fair point, but I do have a slight objection to your statement. It’s not possible that they have always been this way. They are creatures that evolve, just like all living beings.

(Now that I think about it, even non-living stuff change through time)