The point of the whispers are for the person to know that you are not able to reply at the moment. When you die, surely you can take the time to reply to him or her. They took the trouble of writing you.
Yeah but that's the thing, the fight is not over until the boss is actually dead or there was a complete wipe because there are many things that might keep a player busy even if he is dead, to cite a few:
1 - He might get combat ressed and back into the fight
2 - He might be the raid leader or similar thus he still needs to concentrate on keeping an eye on what's is going on, doing shouts and so on and cannot whisper straight away.
So it would be really nice to have the whisper back (Or maybe an option to do so) only when the boss is dead or party wipes (And if not, at least rather than saying "combat is over", have the status of it as well, like "boss still at xx%" or "boss has been defeated" or if I was the last player to die "Party wipe" and so on).