Where is Katarina's counterplay?

I know the obvious response is 'just interrupt her ult'. Thats all fine and well for early game, however mid and late game by the time you interrupt her ult even with godlike reflexes, its probably taken at least 50% of your health.

Katarina's ultimate used to have a 'cast time delay' which was removed many patches ago. The change is below.

Cast time delay before channeling removed (reduced to 0 seconds from a 0.25 second delay).

I think its time to reintroduce this cast time, or otherwise decrease the speed at which her ultimate deals damage.

I dont entirely understand why a 'channeled' ability seems be doing over 1000 damage almost instantly.