F-35b and Harrier have so many drawback in performances
F-35b drawbacks in performance are frankly nigh-negligible. Somewhat shorter range is less important for its role(and not like penalty is critical, especially for a forward-based asset); lower G rating - horrible, but whatever.
It's too big for what USMC would really needed, and rather than plane for USMC it somehow turned into USMC for a plane, but this is beyond the topic. Primary users (USMC, RAF/RN, MM) either don't use internal munitions that don't fit inside, or operate them in token numbers.
Harrier 2, for all its drawbacks, ultimately evolved into best CAS aircraft in the world. No, not su-25 and not a-10.
But where are these FOBS for China? I don't see in near future where China will have such FOB.
It will very easily turning political, but let's carefully say that
any beachead, beyond a short frog leap, anywhere is in need of local air cover. It's quite hard to conduct war in SEA/NEA regions without at least considering these contingencies.
Not taking these eventualities into account, while simultaneously building a second to none - global - amphibious capability, - is nothing but virtue signalling. China's amphibious capability alone, right now, has more displacement than most surface combat navies of the world in their entirety, behind only top-3. And its build up is not anywhere near finished.
Carrying arround China proper is for now a sci-fi area; relying on navy being nailed to that beached for providing aircover isn't exactly best plan, as ww2 history has shown more than a few times. USN managed, for good reasons sure, to abandon
both USMC and US Army through the course of WW2. Those also added some of the most bitter and heroic pages to the USN history itself. Neither is good from planning perspective.
Yes, STOL is a niche capability. It's a niche capability that must be measured against two numbers:
1, oceangoing PLA amphibious fleet is ~410'000t as of now (not even counting smaller vessels and RoRos who are
actual transports for troops). Just behind entire JMSDF surface force.
2, Just PLANMC and PLAAAF together is ~85 thousand troops as of today(peacetime strength).
Niche has to take into account these numbers.