Tiers are done based on how well a champion performs in that role not how there numbers are setup.
That's not what most people consider tiers to be. To most people, tiers are solely determined based on a champion's relative strength in the current game. Rek'Sai is stronger than Elise at the role they both share, so she's higher-tier at that role. If she was nerfed, she'd be a lower tier.
"Tier" is not the right word for describing what you're talking about. You're talking about design elements, but tiers is almost always used solely as a description of balance. If someone is the strongest jungler in the highest number of situations, they are the highest-tier jungler. It doesn't matter what makes them a strong jungler. It just matters that they are the best.
I'm not sure if there is a short work that really captures the concept you're trying to talk about, but "tiers" is definitely the wrong word.
Nerf Rek Sai's numbers all you want she still has an AOE knockup, A teleport, Tunnels, Movement speed buffs, her tremor sense for counter jungling, a long range slow etc.
And she could still suck if she was nerfed enough, which would make her a low-tier jungler. Also, Rek'Sai doesn't have a slow. Her ranged Q is just for damage and vision.
Elise has what? A stun? That's it.
Well, that and a gapcloser, good damage, significant sustain, skillshot blocking, etc. However, if all she had was a stun, she's still been the strongest jungler in the game, which also means she was the highest-tier jungler in the game. Numbers made her the best. Then, nerfs to those numbers made her the worst. That is all that tiers are. A measure of the best and worst.
As it is currently Elise ISNT balanced around any role, she does best in jungle but mainly because she gets destroyed going against most people in lane, that's the only reason she jungles.
That means that her only role is the jungle, and if that is her only role, then she needs to be good enough in it to compete. That can be through providing something unique, but even that isn't sufficient alone. Elise doesn't have any way to either match or beat junglers like Rek'Sai, so she isn't viable in that role.
Also, she has been balanced by Riot around the assumption that she is jungling. They've made it very clear that they expect her to be jungling the vast majority of the time and they expected the recent changes to her to make her a good jungler. She is intended to be a jungler now, so she does have to compete with other junglers. We determine how well she is doing by comparing to other junglers. Rek'sai happens to be a very good comparison because she shares a lot of her strategic niche with Elise.
It doesn't have to be the jungle that she's balanced around, but so long as she is primarily a jungler, she has to be able to compete with other junglers.
This simply isn't true, Even the best of junglers can fall off from being viable at times, it's based around team picks always.
Yes, it is true. The state of having at least one situation where other picks don't best you is what viability is. A champion is only viable if they can be useful in some given situation where other champions wouldn't be.
Champions don't fall off from being viable unless the game changes around them. Rek'Sai is one of the best junglers right now, and will continue to be unless she, her counters, or the game around her gets changed. She will be viable unless another champion (or more likely, a lot of other champions) get buffed enough to replace her in any given niche she has. She will continue to be viable up until the point where she doesn't have any niche that can't be matched by other champions. That's where Elise is. She's worse in every situation than other junglers, so she's not viable.
is the weakest jungler in the game and by no means meta/viable, the people playing her play her solely because they love her and usually know exactly what they are doing, but aren't allowed to win playing a champion they like, and it depresses me just watching them do their best to make that gimped champion work. As of right now, there is zero reason to pick Elise over the other shapeshifting AP jungler 
(she even costs half the IP).