Hardly a thing to worry, firstly the stealth capability & fifth generation features of J-31s are debatable as Chinese keeps things in secret, no one knows where it compares (if it really is fifth gen) to F-22/F-35/PAK-FA.
Secondly, even if PAF goes for it, it will take nearly 15 years for first planes to be inducted in PAF's inventory, i.e. post-2025, until than IAF will have FGFAs in very great nos. (80+).
Thirdly, IAF is planning AMCA as well & options for F-35s are also open (though i personally don't favor it).
Lastly & most important, until when PAF acquires J-31s, IAF will be a very very formidable force with the induction of Rafale + sukhoi-30s + LCA mark 2 + FGFA + mirage 2k (upgraded), so PAF is hardly a worry for IAF, IAF is competing currently (& in future) only with PLAAF.
Now if u want u can post my reply "in that other thread"