I think the thing to keep in mind with this nerf is that Riot's main goal with champion balance is not actually to put all the champions on an even playing field (even supposing that was possible given the different balance demands of solo vs. 5v5 and low-tier vs. high-tier play). Above all, they're trying to make the game fun. I'm just guessing at their intentions, but my read on the situation is this: Riven is a champion who, when mastered, wrecks solo queue because of her skirmish potential. She's also fairly mobile, allowing her a pretty good deal of freedom in choosing her fights (again, especially in solo queue). I think the idea of the changes is to cut down on her mobility (mitigating her ability to engage/escape, which can be pretty oppressive/unfun) and to open up a little more counterplay around the ult (a little more chance to dodge it). In short, the changes are meant to open up a little more counterplay against a champion who's difficult to play against with uncoordinated teams.
Now, do I think the changes are needed strictly from a balance perspective? No. The fact that Riven is basically never seen in pro play (although certainly a bunch of pros mess around with her in solo queue) should make that fairly clear. As you point out, her win rate also is not that high until Diamond, and I for one am OK with the idea of having champions who take some work to master but reward you by being very strong once you have mastered them. But, there are other factors at work here.
For what it's worth, I think this is part of a larger pattern. Katarina, Yasuo, Lee Sin, Nidalee...all these are champions who, when strong, lead to play patterns that are unfortunately quite frustrating for a large number of people. (I personally am not too sensitive to this kind of stuff, but the number of people who still complain about Lee Sin despite his crap winrate and bad teamfighting is astounding.) Eventually, they may be headed for reworks. (Unfortunately, a lot of champions need reworks of some kind...) In the meantime, I think Riot is trying to put their balance in a place where you still can be successful with them, but with counterplay that can be somewhat consistently achieved in solo queue. Unfortunately, this frequently means making them objectively kind of bad.
And for what it's worth, I doubt nerfs of the sort on the PBE are going to make Riven unplayable (definitely worse, though).
