I would argue both the US buying F15 in 2024 and China buying J16 in 2020s are money spent to maintain factories and technical knowhows rather then good investment for the air fleet.
On the subject at hand , Z21 , I for one does wish for a revolutionary design , I think prototyping is good , but wide spread adoption of Z21 would be a wrong use of resources .In a future where the role of manned attach helicopter is uncertain adopting any new manned attack helicopter might be a wrong investment ,since these platforms needs to fit into a picture for the next 20 years not 10 years.
We are at the brink of a new paradigm of how war should be waged at a tactical and strategic level , there is a lot of fuck around in the mean time to find out what works , over investing on any one doctrine especially the existing doctrine is a bad idea.
Sure you fight the war with the army at hand not the one you wish for , but I don't think it is wise to invest into what might turns out to be absolute or worse job-less (think US navy LCS program) is a good idea.
Your view/opinion of the PLA buying J-16 (and to an extent the USAF buying F-15EX) is somewhat flawed.
First, the J-16's procurement began in the 2010s, not 2020s -- we have yet to see for how long J-16 procurement will continue, but when J-16 procurement began in the 2010s (keeping in mind its development would've begun in the 2000s) it had a very clearly defined role to fill a clearly defined gap.
A modern, heavyweight 4.5th generation multirole fighter that had a modern avionics architecture able to carry modern and large payloads and be upgradeable with new subsystems and payloads as they emerged.
The only reason why procuring J-16 may not have made sense, is if they had a sufficiently mature equivalent airframe that could be developed and procured at a similar scale and speed which is at minimum non inferior if not superior to what J-16 could offer based on the Flanker airframe.
For Z-21, a similar gap also exists for needing a heavyweight attack helicopter. Z-10 and Z-19 are both medium and lightweight attack and armed scout helicopters respectively.
The amount of payload, avionics, armour (and/or fuel) they can carry is ultimately much lower than what a proper 10+ ton attack helicopter can accommodate.
Similar to J-16, for procuring Z-21 to not make sense, is if they had a sufficiently mature attack helicopter airframe that could be developed and procured at a similar scale and speed which is at minimum non inferior if not superior to what Z-21 could offer based off a Z-20 propulsion/drivetrain using a relatively mature and low risk design.
Wanting a "revolutionary design" isn't going to happen because no such design is sufficiently mature for the PLA to actually develop and procure on a timely scale. Alternatively, if you want to wait another 10 years before inducting a heavy attack helicopter with a "revolutionary design" then that could work, but then you're also going to go at least another 10 years to continue leaving a heavy attack helicopter shaped capability gap in your orbat.
I agree with the easement that Z21 will be mass produced ,but that what I think it is wrong use of resources.
To rephrase my argument , my point is that the capability gap between Z10 and Z21 is not big enough to justify mass procurement , not because the risk of the program is low , it because even given the off the shelf nature of the Z21 prototype , I think it is not future prof enough.
Every choice has upside and downsides , I might be wrong here ,but I think PLA army aviation would benefit more from more units of an existing upgraded Z10 , instead of the Z21 , given it is unlikely that PLA has also a stealth heavy attack helicopter and a tiltrotor in the coming decades. Basically Z10 is good enough now , and better things then Z21 is coming along soon.
I think you very much underestimate the capability difference between a 6-7 ton attack helicopter like Z-10 and a 10+ ton attack helicopter like Z-21.
As for being "future proof" -- the payloads, avionics, and growth potential in terms of weight are major aspects of why a 10+ ton attack helicopter has growth potential.
It is why AH-64 has continued with new variants over the years, and it is why Italy is developing the larger AW249 attack helicopter which is larger than the 4.6t AW129 (though even AW249 is still a bit small at 8 tons), and why Turkey is developing the T929 10ton attack helicopter even though they already have the 5t T129.
Really, you have set your expectations a bit high -- for the PLA, they don't have the luxury to think about "revolutionary designs" for an attack helicopter right now.
Before Z-21, they didn't even have a heavyweight attack helicopter, which should be viewed as a minimum, baseline capability for a properly competent attack helicopter of the modern era. To ask them to achieve a more "revolutionary design" (which I presume you mean a coaxial +/- pusher prop, or tiltrotor or something like that) is putting the cart before the horse, and frankly even the US which has the most competent and advanced helicopter companies in the world, they are not pursing a heavy attack helicopter with a "revolutionary design" despite mastery and demonstration of so many exotic propulsion configurations in the form of in service designs (V-22) or tech demos (Sikorsky X2, Raider, SB-1) or prototypes (V-280), and having a massive fleet of 10ton heavy attack helicopters (AH-64 variants, and also AH-1Z).
In other words, to take what you wrote here: "but I think PLA army aviation would benefit more from more units of an existing upgraded Z10 , instead of the Z21 , given it is unlikely that PLA has also a stealth heavy attack helicopter and a tiltrotor in the coming decades. Basically Z10 is good enough now , and better things then Z21 is coming along soon" -- I would rewrite it as
"PLA army aviation needs a 10ton heavy attack helicopter because existing upgraded Z-10s will simply not be able to approach the MTOW of a 10ton helicopter, and also because it is unlikely the PLA will have a stealth heavy attack helicopter and a tiltrotor in the near future. Basically, Z-10 is not good enough, and there won't be anything better than Z-21 coming along in the near future".
