Good look finding straight series of NXXXUA registrations these days. With a mainline fleet that will exceed 1000 frames, UA would easily run out of numbers, even if it went to UN/UL/US/UB etc. And what difference does it make? UA has 2-3 distinct registration conventions, while AA and DL are all over the map by comparison.

Besides, the actual registration really doesn't mean anyhting internally. Each airplane has a sequential United fleet number that has some connection to the registration, but is otherwise not relevant.