Why the Rengar rework failed and why they'll never spend the resources to rework Evelynn:
It's simple, the champions themselves aren't the issue. Their design isn't the problem, the problem was that stealth itself is the issue. If you take a look at all the long stealth champions in the game what is the common trend between them?

If you haven't guessed it, it's because they're problematic. Each one of these champions are a significant ISSUE when stronger than intended. This isn't stealth's fault, rather Riot's IMPLEMENTATION of stealth. Riot needs to do a full stealth overhaul and give meaningful counter-play to it at any given point in the game. Unless you completely change the identity of these champs (which they won't do) they're doomed to fail with individual reworks.
for that.