Wow, you're really not interesting in arguing my points are you? Instead you choose resort to a red herring which can only mean you have no evidence to back up wanting him to keep this level of strength other than the ridiculous argument that he should be strong just because he hasn't been in the past. This philosophy does not promote balance of the game. I mean, yorrick hasn't been strong in forever, he obviously deserves to be the strongest champ for a bit. Everyone would fucking love that.
We aren't talking about the mountain of other champs that there are problems with. This topic is about karthus, not lucian, graves, ryze, or annie. I am giving my assessment based on what I see in the stats that says he's probably a bit stronger than he should be. That does not imply that he's a huge problem to the state of the game. His playrate is obviously really low, however, again, looking at the stats, a large portion of the games played on him are played by people who either have never played him or who hardly ever play him. The stats do indicate that it would be prudent to trim just a small amount off of him before he becomes a problem.
Then again, since we're resorting to throwing up other champs to distract from the topic of discussion, I'd like to throw wukong into the ring who was allowed to be sleeper op for the longest time even though riot was on record as saying they knew he was strong, but he wasn't prevalent enough to be a problem. Funny, his play rate never felt like it mattered when he was on the enemy team. This will be the same for karthus if he's allowed to remain as strong as he is.
And a 57% win rate for 5-15, 58% for 15-50, and 60% for 50-125 is absolutely a red flag. Remember how fast ekko hit 100% pick ban when he was that good? Surely you must, it was pretty recently.
Now, his win rate could be due to players playing him into matchups that they perceive as being good for karthus. I do not have the time to look at this into that depth. Personally, when I used to play him, I almost never picked him blind.