A similar thread exists regarding general "what if?" questions about historical events. I have always been extremely fascinated by this subject, endlessly speculating on the various alternate outcomes that might have presented themselves had shit happened differently.
Since I'm also a music enthusiast, I occasionally wonder about what-ifs in music history.
For instance, suppose Syd Barrett hadn't misplaced his marbles and had continued as Pink Floyd's guitarist and chief songwriter into the 70's? How might their music have sounded different? Would they have continued the psychedelic blues pop of the first two albums, or was their progressive direction inevitable? (One might argue that they somewhat came back around to their emphasis on shorter, more radio friendly tunes with Dark Side, the shorter songs on Wish You Were Here, and ultimately The Wall and The Final Cut). Would Roger Waters still eventually emerge as a gifted songwriter in his own right? If so, what sort of tension or power struggles might have occurred between Roger and Syd? I can guess that in this incarnation, assuming there is no David Gilmour in the equation, that a Syd-Roger rivalry would've looked like this: Nick Mason would probably side with Roger Waters, whereas Richard Wright would probably be in the Syd camp. This might then play out either two ways: a) Syd and Roger would share about 45% of the songwriting duties each, with Wright penning the rest of their material. This would be an uneasy partnership between Roger and Syd as co-leaders, but it would be a fruitful relationship, something akin to Lennon and McCartney in the Beatles' latter half. Pink Floyd would primarily be a rock band in the seventies, although there would still be a slight progressive quality to their music. b) The band would've split into two separate entities, one bearing the name Pink Floyd, perhaps the other as a new band or a solo career. Syd's band would continue to write pop, albeit very dark and melancholy pop, while Roger would get to focus on writing about how daddy was never there for him because of the war...of course without Syd's madness, Roger would lack inspiration for one of the chief recurring themes in much of the Floyd's music: madness accentuated by the stress of fame.
In short, Roger needed Syd to go mad and leave the band in order to himself realize his full potential to become a gifted but tortured songwriter.