I understand that resistances feel underwhelming right now, and I'd totally agree something needs to be done about it.
But to say they're completely useless is just hyperbole that damages the actual concern of the state of mitigation.
Armor is still pretty great against lethality. MR is still pretty great for stopping a LB or Elise or something from one-shotting you. (instead its 2-shotting... but that at least gives you time to maybe flash or something)
But yeah... mitigation is in a pretty sad state. The only viable options for survival are

That and playing matrix bullet time all game and just doing nothing but dodging -- which isn't fun.
A lot of people will point to the high damage in the game -- which is probably part of the truth, but not all of it.
I think an equally viable culprit is the amount of champs in the game that have 2-3, sometimes even 4-second CC's that have fairly low cooldowns and resource costs.
If you get hit by anything, its a death sentence.