Congratulations Balance Team: Azir, Veigar, and Kassadin are all below 40%
The community is so glad that these champions, with their less than 50% win rates prior to their righteous nerfs, have effectively been removed from competitive play. It's so nice to see such evil champions thrown into the fires of trash tier.
[end sarcasm]
In Platinum Kassadin Win Rate: 47.83% to 34.88% Pick Rate: 10.28% to 3.91%
Veigar Win Rate: 44.78% to 37.25% Pick Rate: 4.74% to 4.52%
Azir (comparing pre-nerf to today) Win Rate: 48.29% to 37.26% Pick Rate: 6.45% to 1.72%
Urgot (for reference) Win Rate: 37.18% Pick Rate: 0.29%
Now, you may say "but win rate isn't everything." And you'd be right. But it does mean that these champions are making people lose games. At the moment, Kassadin is making people lose 15% of games simply because he sucks so much. Put another way, 30% of the games he would ordinarily win he will now lose. You may argue that things will rebound slightly and they might, but they will not rebound much (see Azir for reference).
Riot, your community saw this coming. We saw it coming and we warned you. But you did not listen. This is disheartening, to say the least.
UPDATE: Some have expressed in the comments concern that this worry is premature. Azir's been out for a patch. On the second day of the nerf, Veigar's win rate barely rebounded and Kassadin's dropped even further.

. Of course, he feels even more useless till late game, but you don't have to try to burst people down with (630 + 2.10Ap) damage.