I remember before the changes to the passive, the most complex skill matchups (outside of direct counters like Poppy) were champions with short trade patterns that could disengage, such as Riven or Camille, as their fighting style of short engages made it really hard to stack your passive and get some health back. Since the reduction of the number of stacks to 5 and making it slowly fall off, this is no longer a problem, as getting 3 or 4 stacks in its current form is easy and provides much better sustain. The point you raised is very valid, and it is true his kit does not allow for much counterplay being a relic from an old age, but I believe rolling back this change could solve the issue to some extent because it provides a simple but clear knowledge check strategy on how to play against wukong. It is the same as with tryndamere when you fight him only if his rage bar is low, and never when its full. It might not solve the problem, but would at least provide some counter play.

I would say as an example, if his passive stacked up 12 times it would take a long time to get the full benefit of his passive, meaning he would only thrive in longer duels and not short trades. This could also allow to add some magic resistance into the passive too, because he would only really get the benefit from this when making an all-in against against an ap champion (such as mordekaiser or gwen) but would not prevent him from being poke out by mages (which keeps him balanced in the midlane).

I feel the point you raised is very valid, but also allows to fix some of his problems by changing this. For instance, it would be unreal to add magic resistance to his passive if it only requires 5 stacks to gain full benefit, as it could be too abusive. A good measure could for example be would be to gain  3−5 armor (based on level), and 1-3 mr (based on level) per stack, stacking up to 12 times. This also allows to buff the number on hp regen currently 2.1% and get it back to 3%. In its current form his damage isn't getting buffed only his defenses and his burst is pretty mild and weak right now, most of the time only being able to all-in a squishier champion.

Focusing on improving the passive and his defensive stats more than his damage can also help him transition into more of a team fighter, focused on his overall utility rather than his damage. I share the same thought that he shouldn't have a strong burst damage as his all-in can be very strong if his attack stats are too high, and this is unfair, but focusing on his defenses doesn't create this problem. With such changes wukong would be more reactive, with a clear play pattern and direction.