4500 range is roughly half Kled's ultimate range at level 6
That range is the perfect sweetspot in which a large majority of midlaners like to either poke or farm. They (assuming this change went though) would know about the damage reduction from range.
The enemy midlaner would either be able to melt you before you can safely ult, be able to walk away due to the ult taking 3 seconds to channel, and if the enemy midlaner has ANY CC they can use it on you and interrupt your ult.
6000 range would make more sense(if you were to go through with the outrageous damage reduction) because then the enemy midlaner has to worry about karthus.
Also you can't have the ult reduce in damage from a farther distance if you don't increase it for the enemy being a closer distance otherwise the attack makes no sense (think pushing a ball on carpet. The closer the ball is to your hand when you push it, the more energy the ball has)
So you also have to add on to karthus's ult change
If enemy is within 3000 units of karthus they take 300/450/600 + 100% of AP damage
Now if someone is dumb enough to actually fight you when you ult like this...
Then we'd still have people complaining about it.
and Karthus's stats don't support the change you want to make to him.
Karthus's state right now is at best average thanks to this meta of "DAAAAAMMMMMAAAAAAAGGGGGEEEEEEEE"
Ironically so Karthus's best part of his kit in this meta is his passive as it allows him to dominate teamfights by dying purposely and then giving the enemy team some "DAAAAAAAAAAAAAAAAMMMMMMMMMMMAAAAAAAAAAAAGGGGGGGGGGEEEEEEEEEEEEE