Wouldnt this be the same case for PAF fighters when S400 arrives
Somewhat, except PAF has had a defensive posture since the 1990s so battle is likely to be over Pak skies rather than Indian skies. S-400 and other SAM systems that India has / will have essentially makes Mirage strike missions very dangerous and increasingly obsolete.
In Kashmir, LRSAMs like S-400s will not have proper coverage as the mountains are highly obstructive. That is an oversimplification as mountains can also be used to create SAM traps, but the situation is less in a favor of SAMs generally than in the flat plains / deserts. So, if there is a war over Kashmir, it will be a little bit of a mixed bag.
Nevertheless, S-400s are a formidable weapon system and will negatively impact PAF. How decisive this will be is open to debate with no fixed answers.