Hi. I used to play Akali, Katarina (pre-rework too), and some Janna.
You usually adapt to the nerfs. If your main is one who is susceptible to nerfs, you can only hope that it doesn't become unplayable. If the nerf is too large, you take a break and then come back, so that the nerf doesn't throw you off. (I had to put off Ori for some time after her AD nerf, because I was too used to her old values).
Pre-rework Katarina was my best champion. After her rework, she was the same, but not the same. I got used to rework Kat. They kept nerfing her. I had to take a break because I was so used to her pre-nerf. I've since lost interest, but I'll probably pick her back up at some point.
I don't consider AP Akali worth playing anymore. If I ever play her, it's tank now. Aside from having to change how I play Akali completely, I just learned new champions. I completely deferred to the support role, and am still trying to learn new champions. I'm waiting for her rework at this point.
Janna - They nerfed her a lot, but not her core strengths, so she's still the same. I'm praying they don't destroy her like Akali.
Don't ever become a one-trick pony. When they nerf/rework your champ, you become tilted, HARD. After Akali S5 nerfs, I just spiraled down.
Numbers don't usually mean anything (damage wise), but when Riot makes your champion clunky
, reduces their core strength
, or removes some part of them completely
, you have to draw the line at some point.