You know what I like about the current tier? Tank busters aren't the only thing that use cooldowns. First phase of E8S, for example, is an awesome instance of where using your ramparts and 30%s are encouraged for auto attacks because they hit pretty hard. You -could- forgo using them, and you wouldn't die per se, but mitigating them is much appreciated and does have rewards for rDPS. You could also look at the add phase of E7S for another such instance.

Point is, invulns becoming the de-facto solution to tankbusters doesn't have to be a bad thing if you look at the context of a fight. Now, a fight like E6S, on the other hand, yeah that is very much so lacking in damage both in terms of autos and tank busters, but w/e, you can't have it all especially for a lower tier fight.