J-31 is 5th generation and J-10 is 4th generation.
Stealth is a major factor in air combat.
No matter how much you upgrade a 4th generation fighter, it will get killed easily by a 5th generation one.
Dont fall for the Rafale marketing hype that it can take on 5th generation fighters with upgrades - lol
I cannot obviously know the mix of squadrons by 2030 in BAF, but I am as sure as I can be that top-brass in BAF are already planning on acquiring the J-31 due to cost/performance/availability reasons.