I wouldn't say shrimps are replacing catfish. Shrimps are popular because they're small and they are good cleaners, produce less waste than larger catfish, easy to breed and perhaps most importantly you can keep them in smaller tanks. The advantage of shrimps is really the size and the ease of breeding in my opinion compared to catfish so its easier to keep shrimps in tanks which have been neatly aquascaped.
That being said catfish does have its place in the hobby. Otto catfish for example is small and is able to eat algae at a faster rate than shrimps such as the Diatom algae. Plecos can also achieve this but their size puts people away as they could up root plants etc once they grow larger. Corydoras will be pleasing to the eye when kept in a large group, they will sift the sand for food while turning the sand which prevents pockets of gas building up, they will also clean the bottom of the tank of eaten food.
Michael.