There was a reason Kennen's passive lastes 6,25s
A few seasons ago, Kennen's used to be a top lane bully, he maxed W and his passive lasted 8 seconds. His w had 6s cooldown at max rank, making him able to easily chain his Ws. The moment he hit you with one ability, you were doomed, his lvl 9 power spike was absurd. Rito decided to nerf him by reducing his mark duration to 6,25s, just enough time to still chain his Ws, but with a small window of only 0,25s. It wasn't just a weird number, now I'll have to put CDR runes to keep the same effect, that's weirder imo. Edit: sorry about the english, it's not my main language and im writing this in my phone. The title is wrong too...
and
each get 10% cdr) mean that a max rank W can easily have a cooldown under 5 seconds. I can understand the concern within the 6.8 context, but 6.9 addresses the changes.