Why Ahri can't have a useful passive?

Her current passive gives her sustain in the lane but it's almost useless outside of the laning phase. Because she doesn't have short cooldowns to be a DPS battle mage and doesn't have high base stats to build tank. She is a burst mage/assassin hybrid she doesn't need that passive what she needs is damage and/or mobility. Reworked Ahri has even worse passive. It can't even be comparable to her current Q ms boost which going to be removed. New passive gives only %20 bonus ms at ALL LEVELS for 3 seconds with a STATIC 9 seconds cooldown and you have to HIT YOUR ENEMY 2 times with your skills for it . It obviously going to be worst passive in the game which "can't even be comparable to AN ORDINARY SKILL'S BONUS EFFECT." Why can't Ahri's sustain be removed so she can get a useful passive? It could be good if the reworked passive's bonus ms and cooldown scale with Ahri's level with the cost of her sustain. She is also less safe and less mobile with the removal of her Q ms boost.