Bands where a member leaves, and BOTH that member and the band become more successful as a result

The Commodores only Grammy winning single "Nightshift" was released after the departure of Lionel Richie.

hard to argue that they were more successful overall after he left, considering that was their last hit and lionel became one of the biggest stars of the 80s

I know but I was listening to that song this morning and was like wow that was the only Grammy they won (& not for "Easy" nor "3x a Lady" nor "Lady" nor lol "Brick House" etc.)

it's a good song

Did we mention Massive Attack and Tricky?

A rare situation where two members of a band leave and both the original band, now a solo act, and the new one do better:

Black Dog / Plaid

Okkervil River and Shearwater are an interesting case. Shearwater was originally an Okkervil River side project for songs which didn't fit on their albums, and for which writing credits were more or less split between Will Sheff and Jonathan Meiburg. I think Meiburg became dissatisfied with this arrangement so he left OR and after that SW became more or less a Meiburg solo project. Since then Shearwater have become much more successful than they were before the split, although it's a moot point whether OR have also become more successful.