See that is the thing. Cass is no longer the Poison DPS Mage she used to be, pre-rework. That was unhealthy because one Q could do a lot of damage, while she stayed blissfully away with her MS boost
Cass is now a Sustained Damage Fighter Mage, with the focus being shifted onto her E, which franky is her more engaging way of fighting than Q-run away Q-run away. By shifting more of her damage into her Q/W you'd be bringing back the unhealthier version of her.
Lastly, Cass is a very late game focused champ. Despite her Q/W not doing much damage early game, they hit harder and better late game, like any late game mage should. Let me show you some math.
Rank 1 Q with about 40 ap: Around 93 damage spread about 3 seconds (Not factoring in MR) 75 + 18
Rank 5 Q with the approximate 1000 AP from a full build or so: About 685 (Not factoring in MR) 235 + 450
Rank 1 E with about 40 ap: Around 77 damage per E (55 + 22)
Rank 5 E with the approximate 1000 AP from a full build or so: Around 712 (155 + 557)
Comparing the two: 685 vs 712
Now let us factor in the fact with two E's, Cass ramps up poison by 40%
.4 x 685 = 274 + 685 = 959
Late game, her Q's are almost at the same power level of a Single E, while her E boosting her Q's damage by 20% - 40%.
So if anything, her identity of a poison mage was shifted more into late game, while replacing early game damage with her dueling potential. By shifting power from her E into her Q, once again, you disrupt this pushing into a good all the time around the clock.
Math.