It’s a pretty simple economics game...

CO arranged a perpetual license for itself on very favorable terms when it spun out SHARES (before that Known as SystemOne and inherited from Eastern when Texas Air Corp raided EA’s assets). Based on that, for CO and now UA, has an insanely low operating cost for Shares, and has largely decentralized things by building new functionality around the edges that talks to but isn’t actually part of SHARES rather than building it into a monolithic environment.

On the other hand while Apollo was conceived by UA it was sold off (technological) eons ago and UA didn’t negotiate such favorable terms for itself, leading to a higher operating cost of Apollo, and IIRC, shortly before the merger UA had contracted to migrate to another platform (Amadeus?) So Apollo was going to be out the door eventually anyway.

To the topic though, my understanding is that Aero is solely focused on the airport ops side of the equation, and fairly routine airport ops at that. Agents can, and routinely do, drop back to native (green screen) SHARES to get creative. I understand Compass is a refresh of the overlay reservations has been using/had available for quite some time — IIRC again — this one came from CO res well before the merger,