I know one of the most prominent voice against this new common platform aircraft on Weibo is @老王RR涡扇花动机. He velhelmly rejected the notion of such aircraft, with one of his main arguments being that nobody is modifying 757s and 767s to be used as MPA/ASW, EW and AEW&C aircraft today, and that there won't be enough demand for such non-civilian common platform aircraft to justify and cover the costs incured during development and production, in lieu of the C919 and the upcoming C929 (which he apparently believes that domestic companies/suppliers will step up WRT the components for both aircraft "soon enough"). He also floated the notion that the CXF project (which is supposed to be China's all-indigenous variant of the C919) would be coming out sometime within these next few years (though he did mention 2027 for some reason), hence negating the need to develop such an aircraft.
Of course, this certainly doesn't mean that I agree with his points.
@latenlazy,
@Blitzo and others have already explained the reasons pretty well.
As for the demand issue - China already has 70-80+ KJ-500 today (and likely still increasing), alongside the KJ-700s and the aging KJ-200s. Sooner or later, they will need to be replaced - Of which, should this new common platform aircraft be a real thing and be introduced in the 2030s, then it would be about the right time to have the next-gen AEW&C aircraft (based on said platform aircraft) to replace them on a 1-to-1 basis by then.
Yes, ~20 years may be rather short for an aircraft, but given that tactical airlifter architectures aren't exactly the best airframe type for conducting high-frequency, extended duration missions (which is typical for special mission aircrafts as per one of the Adorable Whale's video back then), alongside the very real possibility of the designers and engineers assigned to the KJ-500 project didn't manage to fully take the possibility and potential of how the aerial warfare domain would develop and progress in the late-202s, 2030s and beyond into account when they worked on the KJ-500 back then (remember that KJ-500 actually took its first flight in the early-2010s) - I think that swapping these current AEW&C aircraft out for a much more powerful, much more capable aircraft with much greater potential for accommodating future upgrades by then would be the better/preferable option.
(I would deem this to be independent of whether the CXF or this new common platform aircraft can be made available to the PLA first.)
Hence, it is reasonable to expect a need for ~80-100 airframes for AEW&C roles, to begin with - Which, frankly, isn't that small of a number. Then, there's also most of the Gaoxin aircraft types (ELINT/EW, SIGINT, etc) and tankers/MRTTs which would demand such common platform aircraft as well, meaning that the aircraft's production run certainly won't be low.
In addition, should the PLA decide to procure such platform aircraft for special mission roles in the future - I think it can be argued that the PLA would be having different ideas on what kind of performance of the platform aircraft which they would prefer than their Western counterparts based on the operating conditions faced by the PLA being taken into account (desired capabilities, anticipated future upgrades, endurance, range, lack of overseas allied bases, etc.). This means that what the West is currently doing cannot be 100% copied-&-pasted onto China.
Though, one thing - Should the CXF be actually based on the C919, be in the same size/MTOW-category as the C919, and be actually rolled out in time for PLA service entry - Then I think we may have found a more suitable platform for China's next-gen MPA/ASW aircraft. Of course, this doesn't contradict with the need for the new common platform aircraft for many other special mission roles (if such aircraft does exist).
