Why mass rework of juggs/marksmen/mages happen before mass rework of assassins?
Seriously no single group of champions have caused so much balance and game health problems. Whole concept of strictly assassin champion is outdated and not needed in the game, and Riot seems to be aware of that, since in last 3 years only champion released even close to an assassin was Ekko, who still is more of a light magic damage fighter than assassin in sense of Zed, Rengar or Akali.
Assassin players often point out how those champs are important to balance of the game, but there are masses of other champions who can "infiltrate the backline" or "shut down the carry" in much healthier way - i.e. so-called divers - and honestly all champions in the game "punish enemies for they mistakes". Sad truth is that assassins exist only because many players enjoy powerful feeling of easily one-shoting people.
Aside from few exceptions like Kat, assassins also suffer great identity problems - Diana and Akali are known for being virtually identical and take turns in directly outperforming each other. Most of the time there is absolutely no strategical decision in picking your assassin except for picking the strongest at the time, maybe except for taking one of right damage type to fit your team.
So, why are other classes, with champs like Vel'koz who pose literally 0 problems except for slight identity issues or poor execution of theme being reworked first, when assassins clearly need it much more?
iirc. Which scares me because two of those are the tank killing mages, 1 is a tank mage with super sustain which helps vs assassins, and of course there is Vel Koz. How they are going to "specialize" these mages idk but I really hope they don't make it where they are super niche. But it is mostly the immobile part they were going for. 
(even those Garens Q but you know......whatever riot) and then along came 
who is basically another jugger. They're trying to fix mobility creep relevance. If someone lacks mobility, they should make up for it in utility or power without being "toxic". Because apparently sustain, HP shred, and the ability to assassinate as a mage is toxic.