Even though Signas has a great processor, there would still need to be Units mainly for order and for a better understanding of authority, possibly for rookies (if there are even rookies anymore or if there were any at all; just pre-designated Reploids to handle crises such as Mavericks)
Yeah, we all pretty much went WTF as soon as we read that the Hunter Units had been disbanded. That made NO sense to us at all, for a variety of reasons ranging from HELLO, SIGMA? to 'that's stupid, nobody has any accountability then'. Yet, X and Zero are persistently referred to as 'former' commanders of their units, so... what can ya do? We either have to ignore the canon on this one or find a way to adapt the existing model to allow for that ridiculous premise- which means Signas has to become WAY more powerful than he appears in the canon to make it work.
I definitely prefer the Unit model, with X and Zero as Lieutenant Generals- sort of what we see as the layout in X5, where it's clear the uppermost command staff consists of X, Zero, Signas, Alia, Douglas and Lifesavor.
Even if we want to go on that model, and split the command priority among all of them equally , meaning X and Zero got PROMOTED rather than the units disbanded; someone else may have taken over leadership of the 0th and 17th.
That makes sense too. But not 'hay guys let's have one Reploid running everything HRRRRR'