I agree with you here and would add that the marginal difference on UA between F & C is mainly due to the seat; everything else, especially the service, would be equally bad or inconsistent.

On many world-class carriers like LX, LH, NH, TG, their F is substantially superior than their J, which can be good but far from outstanding.

On SQ, the service is so good in all classes than the difference between F & J is marginal. On their 77W, their F & J seats are similar but both are too wide. The main difference is privacy and personalized service. (And the quality of the champagnes and wines.)

CX F is very good; their J is good but can be inconsistent. But in any case, there is no comparison with UA. I thought the OP question was a joke to begin with.