Karthus main here. About his nerf.

I've played Karthus since I started playing the game. He has taken me from Bronze 3 to Gold 2 and I can say I've mastered him. I love him and I really like the fact that he is an unusual pick because people are not as used to play against him as with any other usual mid laner (which sometimes works against you because your team doesn't understand the huge advantage for a teamfight that me rushing in the middle of the enemy team means). This preseason, and especially with the introduction of Death Fire Touch it put him on a slightly better spot but still not something amazing or Brand-level. It felt just fine. He is still a late game Champion in a current early-game meta. This means that when I have lost this preseason, I've lost very badly. And when I've won, it has usually been very worked until I could get to late game. He is an immobile Champion whose only ways to escape a gank are: every 5 minutes and Wall of Pain. The nerf to his only tool to have any chance to escape a gank is not understandable. The nerf to his only tool to land a couple of well put Qs is not understandable. Do you even know how hard is to land those Qs on mobile Champions? And most of the time when you land them they hit a minion so the damage is halved. Wall of Pain was sometimes the only moment you could land a couple of isolated Qs on a Champion like that. And even then they usually still have some mobility left to run to safety. And then almost every other mid laner outranges him and pokes him much easier than he can. Even when you try to just farm instead of poke the enemy, Lux, Xerath, Azir, Ahri, Zed.... Almost every mid Champion is going to poke you far more than you could and poke you whenever you try to farm.

So I think this nerf only responds to one thing: you don't want Karthus to be a viable pick because he is not a flashy Champion like Lee Sin. He usually farms and farms early and you don't want that on your LCS. Last time he was picked on LCS he got 400cs. And you don't want that.

Just admit it. Karthus is already a rarely choice and he has a not-too-bad/not-too-good win rate. There is no other possible reason for this nerf.