The pre-embark ban has very little to do with guests and everything to do with the crew. If the crew is sick, can't work or has a big outbreak, cruises get canceled, staffing levels drop, services suffers, people gripe and moan about it taking 30 extra seconds to get coffee, etc and blast the cruise line in reviews.
You catch COVID on a ship, you go home, (and now) even if you do have COVID you just hop on a plane and fly home. Their hands are washed. I don't foresee it going away for quite some time given their already stretched crew levels.