i don't think you understand what i'm saying. prepatch, the only cost effective ways to build MR were 1) aegis on the jungler, 2) negatron, 3) GA (barely any). you won't see any of these items during mid game except for maybe the aegis. this was what made the AP assassins so strong. now they can't do true damage mid game. they can't snowball off of midgame as well as before. endgame they are actually worse off because of bulwark, even with the new penetration system.
i don't get the whole "black cleaver makes some assassins viable so it's ok" argument. TONS of assassins were played. the two top teams at IPL5 (WE and Fnatic) as well as M5 had strategies that revolved around assassins. what more do you want?
Bulwark is the only "good" item, and even then it's a giant chunk of change.