I mained Garen before 5.16 and I have to disagree with nearly everything you're saying. Garen was never "immune to CC" -- it was his greatest weakness, at nearly every stage of the game. Yes, he could become incredibly tanky, and he only needed one damage item to annihilate squishies; but he had and has only one gapcloser, and a very poor one at that. His ability to get kited is legendary; a smart Garen will wait until he's targeted with CC to pop Q, but then nothing is stopping more slows from being piled on top of him, completely shutting down his gapclose. Not only that but he brings almost nothing to a teamfight.
I haven't played as many post-rework Garen games as I would like, so I won't comment on how he performs now; but from what I can tell he functions basically the same.
But seriously, you are so wrong about Garen. Yes, he's a faceroll champion, but that's what he is meant to be. Garen is about brute forcing your way into the enemy's composition, going hard as a motherfucker 24/7. That is his essence, his lifeblood. I don't even give a damn whether or not it's healthy, because the fact is that it's fun; it's so refreshing to have a champion that doesn't rely on Korean-tier mechanics or even all that much strategic decision-making, but instead on having the courage to dive balls deep and win through attrition. There is nothing wrong with simple champions; in fact, simplicity is the key to brilliance. There is more to this game than making plays, you know.