Well, to start this out, we can't use the "current" meta as a indication of how a meta where tanks are viable or strong would be. We're currently in a meta where Damage is king, but in a meta where Tanks are now back, we have to consider what that would do to the meta.
ADC's weak or not become mandatory again, as they are the ones best suited to shred a front line, Enchanters and Peel Supports take back over as supports over secondary Mid lane Mages, because ADCs are again the end game carry, they need protection to dish out DPS. Mid lane sees a slight change due to Assassins falling out of the meta do to it being super hard to kill the ADC with Tanks, Enchanter/Peel Tanks, and what they are being dropped out for Zone/Control Mages all being able to basically protect a ADC in some form or another. Assassins will leave mid, and Zone/Control Mages or Support Mages will rise.
Jungle and Top lane are the interesting switches because obviously in this senerio one has to be a tank, if not both. Champions such as Elise, Kayn, and Lee would still be viable in the Jungle, but their builds drastically change. They'll build some offense, but most will be tankier items, because they now need to survive their dives because they won't be able to all out kill the AD target anymore. Mostly though we'll see Sej and her beefy friends resurface. Top lane will generally be a noodle fight again but could see champions such as Gnar, Fiora, and GP come back up because they offer great dueling potential early and great Split push later (not so much GP).
So with this all in mind, as history as already taught us, this is how it would go. Juggernauts could come out of the woodworks (S5 worlds-S6 anyone?) because they offer plenty of damage and almost as much tank as a tank. If a tank lost chunks of damage (their damage really isn't that high to start with, specially once mid-late game starts) we would still never see tanks, thus none of this entire post would matter.
Think of these two teams
Gnar - Sej - Syndra - Cait - Thresh vs Mao - Lee - Ori - Jinx - Bruam. Two generic team comps that we've seen years prior.
On Team 1, Sej is most of the engage, Gnar and Thresh each could engage but Gnar is somewhat random and Thresh would be better suited protecting Cait from Lee and Mao engages with the help of Syndra. As it stands now, Sej as some damage, enough to push Jinx off but usually not enough to kill a Jinx later in the game, specially one with a Ori and Bruam protecting her, so to take more damage from Sej would basically disway her from wanting to even try to make it to Jinx, assuming she could even get to her in a front to back through a Mao and Bruam while Lee would try to insect the Cait into his team and then more then likely get brought down,, but then theirs also a Thresh who can Flay, Box, Hook to slow him down or downright stop him. Syndra can QE. , and Cait herself has her net. If we take damage from Sej and Mao (The main two tanks in this game) we'd end up with a game of Sej vs Mao and who can survive longer. If Sej did make it to Jinx and managed to CC her, yes Cait could start to put damage down while Jinx can't for a few seconds, but would it be enough?
I get that in theory, you are correct, but in practice? I don't think you are. Playing this game since Season 3, I've seen a lot, but I haven't seen all the classes in League all be able to play together, the reason is because I don't believe they can. Someone is always gonna be screwed by the meta, currently that's ADCs and somewhat Tanks. If Tanks rise again, Assassins and Bruisers get hosed.
That's just my two cents.
or from runes themselves, we CAN'T let tanks surpass every single champions in every single area once again. We CAN'T let conqueror, true damage and penetrations ruin the game either, so if we want to REALLY fix that problem, a lot of changes must be made and people shouldn't be surprised by it; its not just a "remove conqueror and then game is balanced" situation.
