Why you don't need to worry about a Tank meta

To preface, let's talk about the Tank Class Update. Unlike the other class updates, one of the intentions of this one was to limit the power of the class in question. How they accomplished this was by adjusting the items so they were more effective agaisnt the intended damage type, and less effective against the opposite. That is, HP+Armor items were adjusted to be less effective against magic damage and vice-versa, since the Total Effective Health against the opposite type is just the base health of the items, which was reduced. Randuin's had its base stats lowered so it could be made much more effective as a counter specifically against crits. Adaptive Helm, similarly, has lower stats to balance out that it's far stronger against spammable spells.

With that in mind, any Tank meta from then on is going to have a far less oppressive advantage for them than when any other class is the meta. The way Tanks have been adjusted to be weaker against diversified damage types, a diverse team composition will always be an easy solution.

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And even if we disregard that entirely, there's still the fact the ability to ignore defenses has also been buffed. To counter attempts to build defense, we have: (for physical damage) and which were both buffed
(for magic damage) which was buffed and to increase it further even if you don't have your own cc,

And there's still Conqueror.

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What I can see if tanks become meta is what I and others have been asking for-- longer games that aren't decided in the first twelve minutes. But unlike metas where one class simply dominates in damage, it won't be anywhere near as oppressive to other picks.