"Personally, this seems to indicate to me that the Yuumi changes were not successful at all. Wasn't the entire point to have her be playable for casuals without being too strong in pro?"
It's worth clarifying that Yuumi had a 45% solo queue win rate and like 80% presence at Worlds last year. Getting both of those to move in the correct direction is a success. She's currently 50% win rate in solo queue and trending toward ~85% presence in pro.
So if she winds up 47% and 2%, that's a marked improvement from 12.18. A partial success is better than nothing at all.