I agree that the attention of camera makers is concentrated on full-frame. My attention, not. And I believe that many of the older DX/APS-C DSLRs are still in use, making these, perhaps, the bigger photographic-gear user-base (although not the bigger buyer-base, after all, there is “nothing” to buy).
I “do not know” what an entry-level system is. I know what a lower-end system is: a system that, potentially, is less expensive, smaller and lighter and, at extremes, with less image quality. This difference is important to me, the lower-end system should not need to be less developed, have fewer features or buttons.
Surely, I could imagine APS-C as an “entry-level” system, if e.g. a consumerish built, but decently performing, lower-priced, FF-capable 35mm prime, would, almost be “offered” as a kit, with a Z50. Later, would come the “advertising”: "Do you like your system? Now imagine it with a bigger sensor and IBIS: it’s a Z5, you do not even need to buy a lens, your present 35mm lens will be perfect for street and family photography". I suppose it is not being done.
Really, I believe, APS-C works better as an accessory (not an entry) for FF. There are always days when even the most hardened FF user will need more reach or desire less weight, and FF lenses are compatible with APS-C, not inversely. But, the lake of IBIS in the Z50 will not be welcome for this purpose, nor the lake of direct access buttons (well, the AF-selection button was removed, even from the Z6 and Z7! But returned on the Z9 and 8, didn’t it?). I really believe that a Z60/Z70 could be the most consensual companion for a Z8 (but other would want a Z90).
I do not know if the shrinked-market really makes it impossible for the camera makers to improve APS-C and FF systems side by side. I would, from the user’s interest point of view, try to contour this difficulty with longer iteration cycles and as much parts sharing as possible. Also, I do not understand why a lower-end FF camera can cost almost the same as an APS-C camera and have simultaneously a big margin of profit that the APS-C camera would not have.
Personally, nothing of this matters to me anymore. As other have said, DX was (after film, of course), my entry and will be my exit system (most surely). So, I do not buy anymore (besides, perhaps, one or another used lens). But a well-made D7800 would be an enormous temptation. (Using the same sensor and on-sensor stabilization of a Z70, these two could become a simultaneous launch hit). As would be a few smaller F-mount cameras, even with still smaller sensors and even mirrorless… the F-mount is still the biggest problem, for me, (even if I still like an OVF).
Someone has asked here somewhere, where could camera-makers earn money with APS-C users, well, if there are as many DX users still active as I believe that never got the lenses we wanted, APS-C lenses is a good place to earn some money. I could give you a big list, but I will name only my top two lenses (for which I keep looking for the best existing alternatives and cannot convince me to buy any). First, imagine a Tamron 18-400, without the 18-55 range, but with better optics (also a tad brighter, f/4-5.6) and the performance of the full frame Nikkor AF-P 70-300: meet the (Tamron) 55-400mm f/4-5.6 lens, consumerish built and light, but with a proper focus motor and well performing VR. Second, with the same consumerish and light built-philosophy, but enthusiast performance: a 55-200mm constant f/2.8 lens. And I’ll give you a bonus third lens: a 14-40mm constant f/4 lens, (if someone else would like to make a competing 13-35mm f/4; I would have great difficulty to choose). All F-mount DX lenses. And everybody speaks about the usefulness of the 18-140mm, when the right lens would be a 16-135 f/3.5-4.5 lens (yes, I have the 16-80, but sometimes it is too short). Well, I could go on and on. DX is dead, but not DX users… yet!