Indeed it happens but the smaller luxury lines like Seabourn and Silversea are much more likely to try to accommodate passengers than the mass market lines. In fact Silversea has a fare guarantee program that extends any fare cuts to already booked passengers. Of course what they do is rather than cut fares they’ll throw in other incentives like free air or pre-cruise hotel rooms and those don’t automatically extend to booked passengers. When that happens I have sometimes been able to get something like a slightly offsetting OBC. Will be interesting to see what RCCL does with Silversea now that they control it as they’re bringing a whole different set of fare management practices. As you said YMMV.