I don't think it was THAT reason.
If i remember correctly, she was gonna be hard to balance (in other words, hard to be put in a place where she was either viable or not [according to riot ofc]) due to her poison mechanics.
I was not a cassio main, but i did played her a lot, even had a skin for her, she could get easily first blood at lv 2, her passive was ok (with a
and the correct management of it, she was practically manaless) and she did A LOT of dmg with her Q/W, hell and when you had
you could literally walk though miasma and take half your HP if you were a squishy.
The only thing she needed was maybe a buff on her Q (maybe a but more ms?), hotfix on her E and some tweeks on her ultimate (it still feels a bit clunky), but they really fked her up.
I'm pretty sure a couple of hotfixes would have helped her a lot, seriously, she wasn't popular because she is not in the meta, no mobility, no popularity (at least nowadays), riot wanted to do something with her and unfortunately they fked her up.
But anyways, what if she was a lane bully? She has no escapes for christ sake, a lot of other mages work the same way. I wish i liked malzahar instead of cassio, why? because they already stated malzahar is sitting in a "good place", but his pick rate says other wise, but nooo, they wanted to rework (aka fk up) cassio.
I liked her playstyle, as you say, IF played correctly she was a lane bully, but due to her AOE mechanics (you either need to hit Q or W in orther to follow up with E) you would kinda push the wave, so you would open up to ganks. Her dmg i think was "OK". In other words, she WAS FUN, now? they turned her into a stupid farm mid laner, like nasus top lane or veigar who either gets ahead, or stays behind.
Wish i could get a free refund on my cassio skin tho.