Tanks shouldn't be able to stall just by living. If they can, someone's doing something wrong. They should need CC and some placement, and be aiming to set up kills and soak cooldowns.
There's probably a lot of problems concerning armor items as they have been scaling down and MR items have been scaling up. ADCs probably feel more powerful as the items constantly lose numbers and AD items gain numbers. Randuin's just got weakened too, and Tabi are pretty much necessary. Since tenacity doesn't lower airborne, and that's the popular cc, Merc's aren't exactly giving Tabi competition. Tanks probably feel poor because ADCs feel so strong right now.
Armor is in a really shitty place right now, but mostly because it suffered reductions while AD benefits from all sorts of number-uppage; Mortal and Bork are just two examples of items designed to give ADCs the edge over attrition. Their sustain hasn't really been better, and because damage is lumping so hard on items, it's not like damage and sustain are exclusive for them; even goddam Bork is sitting at 40 AD now. It's pretty much a mandatory rush on lots and lots of ADCs. I remember when it was more situational. Most ADCs that don't garner passive AS instead have damage multipliers or additives tucked away. Yeah, Bork lost AS-- but it just got it back in damage, and AS isn't a super hard stat to get for an ADC as it is. And it still gives 25%, which is hardly 'bad.'
I don't know, ADCs seem to be riding the wave this season. There's a lot going their way.