Rear Echelons Role would quickly lose their role as any role they fulfilled is quickly replaced by units better at it, Assault Engineers and Riflemen
But I do personally like the sound of this would have been better if USF was designed like this
For the Mechanised Doc I think Cavalry Riflemen replacing Assault Engineers would also be better
RE is still the best zook platform for USF and they scale way better than AssEng. Also early games they are the only scout/sweeper for your rifles.