In my view, a psychological factor also plays a role here. The X generation certainly offers everything you need for production, but do I want to spend that kind of money on a new X16 (and its „dated“ chips) when it has been on the market for several years and we can see clear that something has changed with the native porting at UA? I'd rather buy it "used" and save a lot of money if I need something from the X-Line now for production reasons.
I think there are reasons why Apollos are offered with additional accessories (Sphere Mics, Satalities) from time to time. You wouldn't have to do that if the demand was high enough alone on Apollos. But I have no concrete insight or technical understanding of what or why has not yet developed the Apollo series further. I am very pleased with my Twin X and X6, came back from Avid Carbon and would love to see new Apollos and get them in a heartbeat.
Maybe they are at an impasse and it will take a while until there are enough native plugins to open up a new era. Maybe they are already developing a new DSP-format while porting UAD2 to native in the background.
And I think there is room for improvement, as some have already mentioned here.
Whether it has to be the converters...(hard to believe that this would be the main selling point for a new Apollo line)
Room Correction like Audient is currently doing in co-operation with Sonarworks or an own UA variant is in my opinion a missing link in UA-Unisverse. (But please don't forget that option for the headphone outputs)
Dante/MADI would be great for at least 32io per device (preferably as an optional card)
Last but not least, console update for bus routing and hardware fader controller (possibly also iPad) without using limited 3rd party options. Orientated to the SSL 360 would be nice.
I know - Luna has all this and it's great what has been created in such a short time, but I don't think you should ignore all the other DAW users who would like to use Console as a front-end.