In terms of defending Pakistan air space PAF is quite capable of it and will be in future, as new advance systems are coming online, the issue is deep strike, SEAD/DEAD against very capable Indian air defence and here fighters like Su-35, EFT-T3B comes into consideration they both add robust strike and air defence capabilities but when it comes to J-20 it's a game changer in sub continent because even if IAF get FGFA or PAK FA the PAF will still be able to hit India hard with surprise element like 65, it will allow PAF to go for preemtive strikes which since 90s they can't do.