I have posted before about chemistry and how it effects cohesiveness.
Duran Duran started with Stephen Duffy as the singer. I like Mr. Duffy but he is not like Simon and the band would have been drastically different.
Some times when you take out even a single person from a work place, a band, a team; the magic just heads to the proverbial loo.
Here is Duran Duran doing Say a Prayer.