Karthus got a considerably good rework about 2-3 years ago. He still plays well, and he has a pretty high skill cap.
He takes time to master, and you have to be good at reading opponents in order to use his Q effectively. Contrary to what you think, Karthus has immense team fight presence, whether it's from Requiem or whether he's running into the fray with Defile. He is the single best champion for causing an enemy team to splinter, and his death only makes them run even harder. This means your team is better equipped to take out the scattering enemies and get the upper hand.
The problem you're running into is that he probably doesn't fit your playstyle. You may like him thematically, but not liking how you play him is not grounds for needing a rework, especially when he's plenty effective as he is now. Just learn to time his Qs better and when to throw down in a fight, and you'll see how strong he is.