Instead of building 400 J-20s, China can build 200 units and use the remaining money to research and develop the J-31.
Depends if the J-31 is useful. Seems like it'll become a naval fighter only and PLAAF has no interest in it. Now if it were a simpler single engined stealth fighter, it could make sense but performance for single engine in the F-135 category is just not even close to being available yet. Would be better to take the money for 200 J-20 units and divert it towards 6th gen and other areas not 5th gen because that's done. Even improving the J-20 would be a better use of funds.