Good thread that makes good conversation.

To clear up what you touched on with the follow up to "My Guy","When I'm Gone" was the sure fire follow up and "Whisper You Love Me Boy" was to follow that. Both the tracks and vocals to "Whisper" were recorded in the same two day period as "My Guy": March 2nd & 3rd 1964. As for the proven producer getting the singles:that was in effect as long as what other producers brought in to challenge it fell below it in quality. If someone else's song was stronger than yours they could very well come out on top,seldom did that happen though.

To answer your question directly, Mary would have gone on to have major success. It would've been very interesting to hear Smokey keep her in tune with the times. Furthermore, listening to her sing on a Motown record with the sound at it's peak in 1966 is also great to consider. Berry Gordy's plan was to fill the vacant slot with Brenda Holloway, but due to politics and a lack of "cooperation" it didn't happen. Not spending more time in Detroit hurt Brenda in my opinion.

"Whisper" is a good tune, but I think Smokey would've come up with a catchier song in the same vein and got it taken off the release schedule. The Supremes probably wouldn't have gotten the promotion, Mary's leaving moved them up the line a bit. In fact, Motown had to beg Dick Clark to allow them to join his tour which gave "Where Did Our Love Go" room to breathe.

All in all, Mary shouldn't have left Motown. Hiring a lawyer to negotiate a contract with much better terms after her 21st birthday was the way to go. She was too emotional and Herman Griffin didn't make anything better. He got her more money than she knew what to do with at the time only for her career to stall. In addition to that she signed damn near everything away to leave so her decision was costly in the grand scheme.