Personally, if J-21 turns out to be as good as any of the other 5th gen twin engine medium fighters (ATD-X/KAI KFX, AMCA, TFX etc) then I would be quite happy.
I don't think any of us expect J-21 to be "amazing". It probably won't be super manouverable, certainly less so than J-20, it should be stealthy from the frontal, and hopefully, side and underside (as opposed to F-35 whose stealth in those aspects are less than satisfcatory with its bumps and humps), and equipped with the sensors and sensor fusion one would expect from a 5th generation fighter (AESA, EO/DAS type system). If it can achieve a near F-35 combat radius and carry 4 AAMs internally along with all that then we shoudl not be dissappointed at all -- if future iterations are equipped with more powerful engines (not an impossibility, depending how much it ends up weighing and the projected WS-13B engines suggested) can allow for some level of supercruise then it will be amazing.
I think the excitement is arising mostly from the fact that if this thing makes its first flight in the next six months, it will be the second stealth fighter china will debut within two years, something few of us would have even considered a scant possibility three years ago (and probably something western "analysts" will decry and try to diminish even after it happens).
What you have said is all perfectly good and valid, and fits in with my expectations for the internal payload and basic characteristics I would expect the F60 to be capable of.
However, the thing that bothers me the most about the F60, appears to be the thing you have not touched upon - it's purpose. More specifically, it's purpose and role within the PLAAF.
What you have described is a low-end 5th gen air superiority fighter. Why does the PLAAF need that when it has the J20?
Of course the PLAAF cannot just buy and operate J20s, that would be far too expensive and they would end up with too small a fleet. But why does the low end of the hi-low mix have to be a 5th gen? The J10B would be more than capable of forming the "lo" part of the PLAAF's hi-lo mix for the next 2 decades or so.
The F60 would be very useful to the PLANAF as it's carrier borne 5th gen air superiority fighter, and also as a far cheaper alternative to the F35. But, the PLANAF order is unlikely to be substantial, and few countries could afford to field a 5th gen for a single mission.
If SAC designed the F60 as purely a carrier air superiority fighter for the navy, it might turn out exactly as you and others have described, and it could become quite a decent bird.
However, I am not sure SAC will be content to just sell one or two hundred to the navy and massively limit the export potential of the F60 by making it a pure air superiority fighter.
The only way the PLAAF would have much interest for the F60 and the way to maximize it's appeal to potential foreign buyers is to make it a multi-role fighter with a very robust ground attack capability, which leads to internally carried air-surface weapons, which leads to a requirement for large and deep weapons bays and that is where I see this bird getting into trouble.
We don't know too much about the design of this bird, but we know it's generally in the same ball park as the F35 in terms of size, that it is twin engined and has cheek intakes. The F35's single engined design allows for more free internal space for fuel and weapons bays compared to the F60, but even that had to make serious concessions to stealth by allowing all those humps and bumps to accommodate bays bug enough for air-surface weapons and what not.
If the F60 is to be AA only, it might turn out as you hope, but if SAC tried to add in multi-role capabilities to try and grab a big PLAAF or foreign order, then they might have to make concessions and compromises and suddenly it's the same song and dance as the F35 show.
The PLANAF is growing with the Varyag and new domestic carriers to come in the future, but it will still be tiny next to the size and spending power of the PLAAF. It is a bit of a niche market and I do not see SAC content to being relegated and limited to such a domestic market.
I believe SAC will try to seize the 'grand prize' of large PLAAF orders, hell, they might even secretly hope the same thing that happened to the F22 happens to the J20, whereby the orders for the heavy stealth are drastically cut in favor of more lighter stealths that SAC makes.