This is very true, but my thinking is that cost is almost all front-loaded - once we get equipment up there to accomplish the mining, the launches in support would not be terribly frequent, I think, right?

That's possible, depending on how much fuel will the operation consume when in space - and where it will come from. At the moment, I think we would need to send the fuel to move all cargo from Earth. Being able to create it directly in space would change everything, of course.

Land ecosystems had evolved with a fair amount of change. They can recover also much faster because they are "high energy" machines. In the mud you have a lot of hidden structure that is basic for the marine living things inhabiting those ecosystems. Just stirring the mud you can kill a lot of the biomass. And there is the anoxic nasty stuff also that would poison the area.