The RETs would certainly aid ATC during the morning rush, but no doubt a 'cost benefit analysis' would rule them out? A 90 degree turn off the runway requires the landing to virtually stop before making the turn, whereas the RETs are designed to allow an aircraft to maintain a good speed until vacated. All grown up airports have them and it means to can compress more runway movements into a given time period. As you say, the current taxiway system probably would not be able to cope.