Incorrect. Thanks to the new Cinderhulk, Sejuani deals way more AOE damage to monsters than her W ever did. Cinderhulk destroys mobs after level 5, and as you stated, before then Sejuani is better.
Do you really want to argue with me on this? Do you really think you know more about this than I do mate?
Sejuani's old W was 40+3% of your max HP/second+6% of the monsters max HP capped at 300, a cap it often hit. For W to equal the DPS of level 11 Cinderhulk on live Sejuani would need a grand total of 866.66 total HP, ignoring the %damage on W and the fact that Cinderhulk existed and did damage to creeps before the tank update. Also her Q did more damage before the rework and her E was AoE.
No matter which way you slice it Sej's clear now is significantly worse than before. She used to clear out camps in the duration of her W, 4 seconds. If you can clear out a camp in 4 seconds with new Sejuani at all I'd be impressed, I'm pretty sure it's not possible outside of some sort of Runic Echo's/AP cheese on Raptors or Wolves though.
Sejuani is immune to slows and has fantastic synergy with Iceborn Gauntlet's slow thanks to her new passive.
That's not a niche though. Being immune to slows is useful, but it's not something that justifies picking a champ. Also IBG's a subpar item on Sej due to its middling armour value and its lack of HP, while Mana's useful you don't need 500 of it and 20% CDR is almost always overkill. There's just better options at current, be it Knight's Vow, Deadman's or Randuin's.
Rammus is the auto attack reflector, that's always been his niche and it continues to be his niche.
Rammus's AA reflection got gimped in his rework. ADCs no longer even need to worry about the damage, it's far too low to be relevant. Even at 500 armour Rammus's passive deals just over 100 damage, that'll stop em'.
Amumu deals true damage to everyone around him, no other tank can do that. He also deals that true damage based on the magic damage dealt by his team, so he scales well with burst mages in the same way that Sejuani scales well with divers.
I'm actually not going to argue against Amumu, I think he's one of the better designed tanks at current.
Gragas and Maokai are actually rather similar now after the rework with the % health healing passives and the slight knockbacks. Maokai was basically just nerfed so he would stop being picked in the LCS, his rework wasn't intended to help anyone else. Gragas is a very mobile jungler, that's why he's picked in the LCS rather often (and for a tank, his base mana is extremely high so he has far fewer mana problems throughout the game even if he doesn't build items like Frozen Heart).
I don't know if that's the reason why Maokai was reworked, if it truly was then that's all the more reason to revert it. Maokai's rework was the biggest load of garbage I've ever seen and I don't think anyone would even argue against that.
Leona and Nautilus simply have MORE CC than other tanks. Quantifiably more.
Quantifiably you say? Let's quantify it then.
Naut has 1 reappliable Root, 1 displace, 1 AoE slow and on potentially (but mostly single target) knockup
Leona has 2 single target stuns, and an AoE stun
Sejuani has 1 knockup, 2 AoE slows, 1 reappliable stun and a single target stun
Maokai has 1 AoE knockup, 1 single target root, 1 AoE slow and 1 AoE root
Galio has 1 AoE taunt, 1 single target knockup and 1 AoE knockup
Zac has 3 AoE displaces
Cho has 1 AoE knockup, 1 AoE silence and 1 reappliable AoE slow
Doesn't look quantifiably higher than other tanks to me at all.
Alistar has a cleanse, which is actually rather useful for a tank and not something you see very often in that role.
That doesn't at all change the way he plays though. He's generic CC bot with a bit of added durability.
Poppy can stop people from dashing. This is an ability largely unique to her and one other champion in the game, Veigar.
List of tanks who can't stop dashes, Amumu, Leona, Shen. Every other tank can stop dashes. Displacements stop dashes.
Galio is the magic resistant tank, he has MR scalings and a refreshing magic damage absorbing shield, as well as dishing out taunts which hurt mages more than any other class in the game because mages are solely reliant on their abilities to deal damage.
And yet he's still picked into physical teams, because none of that actually matters. He fills the role of generic tank, often better than other tanks so he's picked over them.
Sion has infinitely scaling health and a slow that reaches farther if you knock minions towards people. That's actually an incredibly fun skill shot to play with, in my experience.
But what does he have to differentiate him from other tanks? That's a durability steroid and a lane tool, every tank has those.
Jarvan IV is a diver.
I see him build Stoneplate>Tank quite often. Regardless of what he's tagged as he's filling the role of a tank just as generically as all the other tanks.