There is a big difference between "legally required" and the function they provide.
They were not legally required. There is no rule that requires a caboose. There are rules about critical functions that had to be taken and a caboose facilitated performing them, but they did not REQUIRE a caboose to perform them. About the only two things that would be difficult to comply with without a caboose is flagging and watching for defects. Other than that all the other stuff a caboose does could be accomplished without a caboose (with an increase in delay). Lining switches behind, drop off a brakeman, he lines the switch and walks back to the head end. Getting an air test. Brakeman goes to the rear of the cut, uses a handheld gauge, does the air test, walks back to the head end. Need to make a shove, put a brakeman on the rear and he just rides the shove back on the rear car. Display a marker, hang a marker on the rear car.
What required the use of the caboose was work rules, the labor agreements. Labor agreements also required firemen, they have required a brakeman for every 25 cars, they have required a broom on the caboose, they have required hot plates on Canadian engines, they have required a bunch of things. But just because the agreement required it doesn't mean its absolutely necessary to do the job. Diesel engines don't need firemen. Air brakes means you don't need a brakeman every 25 cars, American engines don't have hot plates.
A caboose was a tool used to accomplish other tasks and comply with other rules. Once the railroad had better tools, they stop using the caboose. The critical functions are the requirement, the caboose is just one way to facilitate doing them.
