The real problem with Riven isn't her animation cancel. It's the inconsistency with which it's used.
When there is such a skill variance with a champ, mechanically, at all ELOs, it becomes impossible to learn how to play against her. I might be playing in gold or silver with a Riven that is "average" in every way for her ELO. I might be able to outplay her. I might develop mechanics that work against her. And then the very next game I might get a Riven who has iron-tier macro (which is the sole reason she is in my game) but diamond-level mechanics because all she does is go into the practice tool and practice combos all day. And magically all my mechanics that used to work.... No longer work, and I get permastunned and die. Because the only reason those mechanics countered her in the first place was because she didn't know how to play her champ.
And any time I queue up for a game, I don't know what I'm up against. Any time I go to gank a lane, I don't know what I'm up against. I have no way of actually knowing whether I can outplay and kill that 300 hp Riven. Because she might have diamond level mechanics and might stun + shield juke me and combo kill me before I even get off a single auto. And she might be in my game because she rage quits every game because "her team sucks." A champ shouldn't have such high skill variance (very very low skill floor and very very high skill cap). At least not a champ with hard CC like Riven. Riven doesn't even need to position. Look at an ADC like Vayne. A very high skillcap champion with a stun. But Vayne has to position good (and Vayne's enemy has to position badly) in order for that stun to work. Riven just needs to stand there. Her entire kit is based on mechanics alone. No positioning, no conditions. There is no way to safely walk up to her and kill her.
I don't know why I typed out all of this. I guess I was just bored and waiting for my laundry. I really don't think "mechanics just for the sake of mechanics" should be a thing though. And that's essentially what animation cancelling on Riven is. It's not like she's actually trying to do something useful (like an ADC orbwalking or kiting). She just has this wonky animation cancelling built into her kit, and Riot decided to keep it and balance her around it (because people like clicking fast and feeling good about it I guess?). And as a result, it makes the champion unpredictable because of the very high variance in skill from one player to another.
