I think one of the main problems we have at the moment is just that everyone isn't up to date.

In terms of the Class updates, not individual ones. So far we've had Juggernaut, Marksmen, Mage and Assassin updates, only one of those to introduce actual defense items into the game. At the moment, we see a lot of posts about Marksmen being stupidly strong against Tanks at the moment, and those thoughts are considerably valid, but consider this. Riot is trying to get a feel about how much Tanks should tank compared to the new offense characters.

I mean, think about it. Each of the class updates have been focused on Damage Dealing Champs, champs that do loads of damage. Yes, our current Tanks are being shredded by an absurd amount of Pen/Shred/whatever you call it, but that's the point. It's easy to make a character with the goal to do damage, but for champions whose entire idea is to take damage, you need to set the baseline for both. "How much damage should this do" and "How much damage should this guy take" go fairly hand in hand.

So while damage creep may be prevailing at the moment, I'm willing to bet money on the fact, that Tanks are going to be much more capable of Tanking heavy duty stuff in the Class Update, simply because, they are now up to date with the tone they wish to set with Tanks, the same way they set damage with Carries. "But why not buff tanks until then", well then you're not setting an apt comparison. The idea around it being "So they tank this much against The New Marksmen Class, as the Current Tank Class, so here is X and X we need to think on improving for the whole roster"

Sure, it's gonna be a while till Mid season, but that's my two cents on it.