That's just not true. Everything you described is about %MaxHP and to a lesser extent about %CurrentHP.
On the other hand, %MissingHP is pretty crap if you go full defense. How do you reduce their HPs to a point where your %Missing starts to kick in, if you go full defense? %Missing is good for executing people, as a finishing blow. It's for burst mages and assassins, and that's precisely who gets it. All those bruisers you're talking about, guess what? None of them has %MissingHP.
Also, just empirically, you can see that other Tanksassins emerged without any %MissingHP damage, so that's clearly evidence this is not the problem with Tanksassins in general. I mentioned Yasuo and Yi in the previous post, but you can also add Akali to that list.