The rub here is that on the OP’s cruise there were less than 200 passengers so basically less than 100 couples give or take. Would enough of the 100 families feel enraged enough to even have a number join a class action suit even if you could find an attorney willing to pursue a small case. Naturally you’d be looking to more than this one instance and you’d have to look to the line’s past behavior and if this was standard practice or a one-off situation. Looking at CruiseCritic, which is not the end all of sources but one of the few, I don’t see any complaints about this. I’d imagine missing ports is relatively rare especially on a river cruise so there may not be many cases to go on.