I imagine non-aisle includes just about everyone who is not a flight attendant (management, reservations/customer care, marketing) -- I also imagine those groups are the ones that have to deal with the brunt of complaints both during boarding and in after-travel encounters (surveys, complaints, "why did I have to check my bag as an elite?")

It's also interesting to think about the "grass is greener" aspect -- FA's may find the process more orderly onboard but in gate areas it certainly doesn't seem any more orderly in the gate areas (if anything it seems slightly more chaotic) and given that the only truly tangible benefit of the lower elite tiers is priority services (including boarding) it makes sense that UA would want to reinstate it.