Auxes can serve other purposes than processing busses: they also serve to process Intrument multi-ouputs (which are something different from busses, as far as Logic's audio routing implementation is concerned.
Similarly, busses (and therefore Bus objects, should you create some in the Environment) can serve other purposes than serving as inputs to Aux object: they can serve to route signal to plugins side chain inputs.
That's most probably why Logic differentiates Bus objects from Aux objects. Should you "merge" one concept with the other, you'd most probably get into compatibility issues.
IFAIK the only thing that changed over time (or the only thing I noticed) is that at some point, Logic started implementing a feature by which routing something to an unused bus automatically creates a new Aux channel with that bus as input into the Mixer. This is only for convenience and worklow efficiency purposes since Logic developpers assumed (rightly IMHO) that most of the time (but not necessarily all the time, as I show above) routing a signal to a new bus is done with the aim to further process that signal (thus the need for a new channel strip in the mixer, need that serves the Aux return). This is what may have started creating the confusion, but the two types of objects are not the same.
BTW, differentiating the two objects enables doing the following, which you would have no way to do (or at least not as efficiently) if you were "merging" the two concepts:
* Take a drum, e.g. kick drum, use that DRY signal to feed an FX side chain (e.g. Bass sidechain compression), and in parallel process that Kick with some EQ, FX, whatever, with only the WET signal being part of your mix...
* Logic's current way: route the output of the Kick to a bus, put all your EQ, FX, etc on the Aux return, and at the same time use the bus as input for sidechain.
* If you were to "merge" the two concepts you'd need two buses: one for "wetting" the Kick, the other (in parallel) for DRY sidechaining (plus you'd have to manually route the sidechain bus to "No Output" so that you don't hear it in the mix). Less efficient.
Logic Pro X 10.3.2 - OS X El Capitan 10.11.6
MacBook Pro retina 15" mid-2014, 2x2.8 GHz Core i7, 16 GB 1600 MHz DDR3, 1 TB SSD
(NVIDIA GeForce GT 750M 2048 MB / Additional display: Samsung U28D590D 27")
Audio interface: MOTU UltraLite mk3 - MIDI Interface: MIDIMAN USB Midisport 4x4
Keyboard midi controllers: Korg M1, NI Komplete Kontrol S88
Additional control surface: Logic Remote (on iPadAir 2)https://www.evernoize.com