With a combat radius of 1000 to 1500km, 300 J-20 would be enough to defend China. You don't need to constantly patrol every single inch of territory, and most critical points are covered by 300+ km range SAM anyway.
1. 300 is a wildly optimistic figure
2. ... you actually propose to use IADS to replace fighters? Are you serious?
3. It takes time to get from place to place, even with supercruise. In any case operational requirements can't be waved away just by drawing big circles on a map. You need to consider org structure logistics sortie rate of airport, traffic rate sensor coverage etc. It certainly isn't good enough to say 'I think so 300 should be enough'.
4. I am not sure where you get the 1000-1500 km combat radius from. Sheer optimism? That's higher than the Raptor's.
5. Eagle in interceptor role can have up to 2000 km combat radius. The USAF certainly doesn't seem to think that's entirely adequate and stop at 300 Eagles. I suggest you look into the reasons of this.
With 300+ J-20, I really question whether China will need any other fighter. Instead, it should shift its focus toward a heavy stealth fighter-bomber, like T-50.
T-50 is a heavy stealth fighter-bomber? Since when? Just because it is postulated (not even remotely realised I may add) to be able to carry ground-attack munitions does not a fighter-bomber make.
J-20 is already in flight testing. I'm sure we will see it enter service in 3 to 5 years. During those 3 to 5 years, PLAAF can keep making J-11B and J-10A with domestic engines.
... three to five years? :roll: I don't think you have a clear idea of how long it takes to develop and test a fifth generation aircraft... or any other advanced aircraft, for that matter. I'll eat my hat if the J-20 enter service in 2015. With any condiment of your choice.
The only indication it'll be about is in the 2017-2019 timeframe, and then it'll be IOC, with very limited numbers - 1 squadron if that. At 1 squadron a year it'll take another 12 years to have a full 300. Even assuming everything in your wildly optimistic scenario (2013 IOC, immediate production, 300 J-20 is all the PLAAF ever needs for fighters) is correct, that's 2025.
So we are back to producing J-10s in the mean time. Are you still seriously telling us that the bog-standard J-10 is good enough in 2018? 2020? 2025? What do you do with the ones built in 2020 or 2023 come the 300 J-20 air force? Immediately scrap them all?
Past patterns in the PLAAF suggests they won't do anything of the sort, so no, we'll be seeing J-10B yet, and if we don't it'll not be because there was no need to upgrade the J-10, but because the J-10B upgrades are not economical or feasible for one reason or another.