im, not sure the problem is tanks. is a darius not able to take down a maokai anymore after he bought a cleaver and a visage? im pretty sure he just gets stacked on the mao and then destroys him. if were just comparing bruisers to tanks, then in extended fights they usually have the upper hand after a certain point in their builds.
ornn is more of an exception, because hes just loaded with % hp damage, which is pretty dumb i agree.
but bruisers usually outscale tanks and i havent had an issue to fight a tank one on one with a bruiser by midgame anymore even if some of them are stronger early on.
the problem is more that tanks are safer and more effective teamfight picks than bruisers, due to how effective protect the marksmen comps are, thanks to just how fucking overpowered marksmen are at the moment. they scale too quickly and too hard, and there is too much potential gold generation on bot lane with turret first blood and the removal of bot lane fortification already since season 6.
the issue isnt that bruisers cant fight or take down tanks in extended traded (yes i know, in the early lane phase tanks often outtrade, but bruisers usually start to outtrade after two items), its more that while a bruiser gets walled off by one or multiple tanks in larger fights, the bruiser gets shred to pieces by the enemy backline too fast and there isnt really an answer for this.
all they need to do is to hit you with cc for a couple seconds which the marksman uses to just blow you to pieces, partially because hes further into his power scaling than he should be.
the tank doesnt need to win lane or get ahead to be effective if the target that theyre supporting is strong enough.
youre correct that bruiser itemization is expensive. but im more worried about how much cheaper and how much more effective adc itemization is compared to bruiser itemization, than comparing bruisers to tanks.