PLA needs the majority of its fleet to have enough persistence at first island chain. Otherwise it may not fare better than Göring's Luftwaffe in the sky of Britain.
J-10 is a good fighter, but it was designed to fight the Soviet Mig-29 and Su-27 coming from the north. Now the mission requirement is completely different. It is still useful in air defense and PLA will buy it. But its rule will be supplemental.
I believe it will more be complemental -- not every PLAAF and PLANAF fighter needs to be able to reach the first island chain striking targets nor will they all be operating in the east. J-10s with drop tanks won't be uselses around the first island chain either.
As for the cost, the right weapon is expensive but losing a war is more expensive. Also, it is rumored that J-10 is not cheaper than J-11.
Not sure about J-10 vs J-11 price, but again, what the PLA needs and what it can afford are different. I agree that a fleet with a majority of J-20s, flankers and strikers would be the best but can they afford and support it? Is it fully necessary in peacetime?
Unless they're willing to downsize there's no way they can be made up of mostly heavy weights, I'm not even that confident J-10 will be the PLAAF's mainstay like J-7 was.
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Basically the crux of what I'm saying is what they want and what they can get is different.
I'd be happy to speculate on what their future fleet could look like and there are a few things which could change between our predictions but something which I believe is near definite is that PLAAF will be sticking with J-10 production for a while, and that its fleet will not be made of a heavy fighter majority and probably not half either (in terms of numbers rather than weight).
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