This would be a nice change for him so that the cooldown isn't wasted on him when he dies midcast. Bard is a really strange champion though, and may be hard to balance because he has such a strong focus on utilities instead of numbers.
That said, if you're dying while casting that spell you might want to reconsider how you use it. I make this mistake a lot too and I'm trying to figure out when and how I should use it. I think it's a great long-range lockdown spell for fleeing enemies or stalling objectives like dragons for your team. Otherwise it's up in the air.