Animation Canceling is a power gate, not an advantage
Now for some truth. Aside from the fact that many champions can cancel part of, or entire animations, and perform some abilities in combination with others. And aside from the fact that anybody can cancel the end of basic attack animations by moving or using an ability. The fact is, it is a gate, not an advantage.
Riven does absolutely shitty when you first pick her up. As you can see a 41% win rate 1-5 games(except right after a patch or hotfix when numbers are thrown into flux).
Meanwhile when mastered she gets up to 57-62% win rates. The thing is. Most dedicated mains get 57-62% win rates on their main champion. However, because Riven has more dedicated mains than anyother champion, due to being fun, strong, fast pace, pretty to look at and having a nice lore, it inflates her win rates.
This is because animation canceling is a power gate. Without it Riven is kind of bad. Like the champion has few tools to effectively be used in high tier play and is squishy as heck, without being able to perform combos and speed up her burst to - hardly more than a fed Divers speed.
If this was taken away. Rather than being nerfed, Riven would require massive buffs to let her mains perform as well as any other dedicated mains. Essentially you closed the gate to her power and now others must be opened.
This means getting back her base stats, some scalings, some tankiness, some real HP sustain in lane. Perhape even a duration buff to her tiny .75 second stun. Meaning, to keep Riven viable for the mains and pro players. She would receive buffs across the board that would make her a nightmare in Bronze-Plat where she averages negative win rates right now.
To be honest. She might become impossible to balance for all tiers of play.
Meanwhile, if she received only a small nerf currently to say her passive, which would bring her in line with good Riven players who know how to cancel and weave AAs. Without hurting her low tier much. She would be in a healthy spot for the most part.
"Be careful what you wish for, in case in might come true."
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