It might be stretching it a bit, but - New Order were every bit as good as Joy Division after Ian Curtis died.

The Doors got much better after Jim Morrison died as they stopped releasing music.

Grateful Dead went through keyboardists like rolling papers. Though they had definite peaks and valleys, the valleys were not the result of bandmember deaths. I will say that the really recovered from Brent's departure, but they were on the way out in general by then, anyway.

AC/DC?

I find it hard to believe, but I'm with Stu on this one.