Alistar being a top pick confuses me. I shred him.
Sejuani I can believe.
And I do generally support this argument that you're making, but when an adc with no crit items, no armor penetrating items, chases a tank halfway across the map perma slowing him around 4 corners, and kills 380 armor as easy as they would any mage/fighter, I have to question tank viability. I literally had this happen as Ornn vs an Ashe, because she had an Essence Reaver. No matter how much I slowed her and E'd away, she could keep up with me because the skills were all resetting on ER. I couldn't turn and fight her, because a Taric was behind her.
Imo, the three tanks I still consider Viable: Taric, Dr. Mundo, and Sejuani. 2 have healing sustain. The other has through wall mobility and armor/resist buffs, and very high damage for a tank.
I don't consider Sion a tank really. He outdamages most champs in the game as a "Tank." Once someone digs their claws into him he usually dies... The problem is how much control he has over turrets.