The J-31 isn't even an official project, so it is premature to speculate on the aircraft's success in PLAAF. What's more, most of the expense of an aircraft lies in the avionics, not the airframe. A high/lo mix of stealth fighters can be achieved with J-20 alone, with one version of the J-20 featuring the most state-of-the-art avionics, while another being the stripped version with only essential avionics. The J-31 is redundant. There is no need to open another manufacturing line or create a whole new logistical support structure for the sole purpose of having J-31 as low-end stealth fighters.
While I agree that it's not always strictly necessary to maintain a Hi / Lo combo, especially in 5th Gen LO platforms, even the US has opted to have fewer tier 1 'air dominance' fighters (f-22) and have (planned) to have a much larger fleet of tier 2 fighters (f-35)...
Many things come into play, some political (lobbying for scarce resources/current political objectives of government), some economic (need to maintain and grow a strong industrial base that is competitive e.g. both SAC and CAC) and some budgetary (how much does the government actually have to spend to achieve its political objectives).
I'm not sure it's as simplistic as, the J-20 is a better fighter, end of story.