I dunno about Riven being mechanically demanding. She is one of the champions that I tell my friends to use when they are struggling to get into Gold because she is pretty easy to pick up once you have the rythm down. It isn't that ryze is hard, he is rather easy.
The problem with his win rate is that a lot of people that were not very good with him spammed him because even while being pretty shit with him they were able to win more often than not. Now that he is no longer over powered those people are losing their games because they never had the actual skill to back up their performance. Once the people that were only playing ryze to abuse him stop playing him the win rate will start to balance out.
It is like if they buffed yasuo to make it so that he no longer needed a knock up to utilize his ultimate. A bunch of people would be spamming yasuo in order to get that freelo. They would all get a tier higher than they were able to get without spamming something that is obviously broken. Then if they reverted the change all of those 'yasuo mains' wouldn't be able to do jack shit in the games they were now playing in since they didn't actually deserve the tier higher rank. You would see yasuo's win rate dive down to 20%-40% easily, even though he wasn't being nerfed, just reverted. Does that mean Yasuo is weak right now because if he was buffed and then 'nerfed'? No, it simply means people saw something was broke, spammed it, climbed, it got fixed, but those people keep on spamming it hoping for the same results.