Should Offense or Defense have the last word in PvP combat?

So in the games current state, it seems like offense itemization being the last step in the counterplay chain is lending to the problem of Zed/Fizz/etc. being really cancer, but I've also heard horror stories of Mundos and Garens that lasted forever in fights when defense was stronger, so I have to ask: which should have the final say?

As it stands, there's people who build damage, and the counter is building armor/mr, to which the counter is penetration, and the counter to that is (unintuitively) build MORE armor/mr, which is finally countered by Last Whisper and Void Staff which both have high % based uncounterable penetration values separate from the actual armor pen and mag pen. Given that offense already helps you more than defense in terms of game objectives (Examples: who cares that you won a team fight if you don't have the attack power to take towers before the other team is back up; who cares that you can solo dragon if it takes you 5 minutes to do so) it feels like defense of some sort should have the final word in fights, even if you boost health over armor and MR there's Blade of the Ruined King to punish you for trying to build defensively.

So should defense have the final uncounterable items, should it be offense like right now, or should just items that are direct super counters one way or another, such as flat % penetration and % health on hit, be nerfed or removed so that it becomes more about who invests harder?