"Which would lead me to my next question, perhaps this is why Miles Davis ’progressed’ into the discovery ( if you can call it that?) of fusion."
A reason (and not the only one,) is that Davis wanted to play music that would appeal to fans of rock music for bigger paydays. I have been searching (unsuccessfully, so far) for the attribution for the following event; In 1970, Miles Davis opened for the Grateful Dead at the Fillmore West. He agreed to do this when he learned how much MORE money he could make filling 2000+ seat venues.
In a bigger scheme, by the late 1960s jazz was no longer "popular music." Steady paydays were getting harder to find for straight-up jazz musicians. This is why you had musicians like Wes Montgomery playing jazz versions of popular hits, and others leaving touring altogether to play as studio musicians, or tv and movie music ensembles.