Rethink that for a second. He has an aoe that drains mana like... I dunno, but you get the point. I still kind of need it late game.
Tear karthus has like 2000+ mana, and it drains like 100 a second rounded up. That's 20 seconds of aoe without casting other spells, closer to 10 with q spam and e usage, assuming no mana regen occurs. The odds a teamfight lasts 10 seconds with a max stack tear karthus standing in the middle of it is pretty slim. And it's not like you need a large mana pool at the end of a teamfight, cause Karthus should either be dead and/or cleaning up the remains with his r.
If you have a tear, you wouldn't need anything else, my point still stands, your mana pool won't be drained completely throughout the fight.