But surely PLAAF don't need J-20 for all borders, only in Eastern coast are crucial. J-20 is not really needed for border with Russia, Mongolia, North Korea, Pakistan, Laos, Myanmar, Bhutan, Nepal and central Asian countries, including Afghanistan ... that suit J-10C or J-11B very well
J-20 is significantly more expensive than J-10C or J-11B
Of course ideally to station J-20 and J-16 everywhere, but resources is always limited and not wise to waste it
You are viewing things the wrong way.
The proliferation of J-20s among various theater commands with increasing numbers and units towards all strategic directions is just a natural consequence of the PLA's modernization overall rather than a specific need to have J-20s face nations with small/incapable air forces.
Because even with more J-20s being built, it's not like new J-20 units will be received to different theater commands in an equal manner. Some theater commands take higher priority than others, and going forwards it is likely some theater commands will end up having more J-20 units than others.
But at the same time, J-20 annual production does seem to now be in the 90-100+ airframe range, meaning it's basically three brigades worth of J-20s a year being built, and the PLA has five theater commands, and the geographical boundaries of theater commands also means that having sufficient dispersion of J-20 units to allow some to be closer to a given theater of potential conflict and some to be more distant (to be less vulnerable to strikes from a given strategic direction to enable rapid redeployment) is sensible.
We already have eight J-20 brigades with pictures confirmed from the latest rounds per our own records and scramble (ETC, STC, WTC have two brigades each, NTC and CTC have one brigade each), and if CAC continues with 90-100 J-20s a year for even another four years (likely to be longer), you'll end up with a situation where each theater command will average four J-20 brigades each, but in practice some likely to have a few more brigades (5-6) and some a few less (3-4).... and if three J-20 brigades (with other 4.5th gen fighter brigades and also force multiplier aircraft units) are in a "lower priority" theater command facing no particularly strong adversary and being massive roflstomp level overkill, well that becomes just a natural outcome of PLA modernization efforts, and can serve as a strategic reserve for other theater commands as well.