Tuning the power of a disable around its avoidability is impossible.
It is fine to balance a morgana snare by very slow projectile speed to make it easy to avoid in a solo lane or a laboratory setting for example. (I'm not complaining about morgana btw, this is just an example)
In this "laboratory" setting the punishment not avoiding morgana's slow projectile is a long duration snare, which is a fair punishment.
But what about outside of that context? What about a real game?
In a real game the punishment is not only a snare, but also a stun and probably any other CC the enemy team wants to put on you because they know you can't avoid it. In fact, Morgana doesn't even need to "risk it" by throwing out her slow snare on a moving target, she can just wait until someone is stunned with a targeted ability that can't be dodged, and then she can snare them while they can't move.
The point I'm making is that "slow projectile speed" in a real team fight in real games is an artificial weakness. It may be a real weakness in a solo lane, but with the prevalence of unavoidable crowd control it can't even be called a weakness in a small skirmish-- someone is bound to have a targeted snare or stun or wide area disable or 80% slow.
In other words, crowd control makes the execution of follow up crowd control trivial, and trivializes the weaknesses associated with particular execution those individual spells. It takes the "skill" out of "landing" a big important spell because the enemy champion / team cannot react since they already failed to react once. This removes opportunity for continual reaction and therefore it removes opportunity for interaction.
Being snared for a precious couple seconds in an important team fight for being hit by a slow moving skillshot is fair and the skillshot was balanced around that case. Being unable to avoid the even slower moving even longer lasting stun that was sent at you without being able to react to it BECAUSE you are snared isn't fair because the slow moving snare wasn't balanced around the idea that the punishment would be a snare followed by a stun.
Cleanse and QSS as a design aren't a good approach to preserving the skill and interaction in the game because it just leads to a path in which the team with "More control" wins. The first round of disables is just to force QSS and cleanse to be used and the subsequent rounds are used to actually create a disable. The fact that crowd control is so prevalent that it can be doubled up without much issue means that the power of QSS for "fixing" a mistake is trivialized, and even cleanse isn't sufficient.
The biggest offender is targeted crowd control followed by slow moving long lasting skillshot crowd control. Something needs to be done to prevent edge case abuse. I do not think that the game can be balanced around the idea that each control spell will be followed up with another control spell, because then each control spell would only last a fraction of a second in duration and leave it feeling horrible outside of this edge case.
We need something that just targets the edge case abuse and nothing else. Something that makes chain disabling a single champion very inefficient / ineffective. Tenacity isn't the answer, because it has the same power per spell whether hit by one spell or by 4 spells serially.
What is actually needed is a systems change that makes it impossible to crowd control someone who is already crowd controlled or has been very (within a fraction of a second) recently crowd controlled. This would make it so that each control spell can be balanced around its own power level rather than the power level of the average control spell, because landing a slow moving projectile is no longer an artificial difficulty-- its a real challenge, since a target that was just stunned via a targeted stun cannot be snared during the stun. In other words, you can't just keep someone locked down who can't react to your choices.
You have to wait until the little user interface control bar beneath their health meter displays the indication that "CC invincibility frames" are done. It could be something like the flashing invincibility frames of smash brothers or megaman or any other game in which the designers value the ability for the player to react to serial threats which weren't individually balanced for serial application.