I can understand Riot nerfing the CD on Maokai's Ultimate.

But why so extreme?

I honestly think a more suitable change would have been making it a static cooldown so that instead of it potentially having a 24/18/12 @40% CDR, it would always have a 40/30/20. That's still better than live with 40% CDR at Rank III. If you made it a static cooldown, you could make it exactly what you think is reasonable. Hell, you could even try creating a new mechanic and make CDR 0.5x effective on his Ultimate.

Right now, with capped CDR, Vengeful Maelstrom's Cooldown is 36/30/24 seconds (60/50/40 seconds with no CDR).