Would the ABB ever have existed if....
If Taj Mahal had never recorded Statesboro Blues with Jesse Ed Davis on slide?
If Duane and Gregg hadn't gone horseback Riding?
If Duane hadn't had a cold?
If it hadn't been Duane's birthday?
Etc.

But, Taj Mahal did record Statesboro Blues with Jesse Ed Davis on slide.
The brothers did go horseback riding which led to Duane's injury.
This led to Duane not playing for approximately six weeks.
It was Duane's birthday so Gregg, possibly trying to mend fences, did buy Duane the Taj Mahal album.
Duane did have a cold or Gregg wouldn't have included a Coricidin bottle with the album.

This led to one of many very good guitar players becoming a legend on the slide.
Duane's virtuosity on the slide led to his session work in Muscle Shoals.
This led to his being offered a contract by Capricorn/Atlantic.

Also, What if the Wilson Pickett sessions hadn't been in the deep south where Wilson and Duane couldn't go to lunch with the other session musicians without causing issues?
They wouldn't have stayed behind. Duane might not have been able to convince Wilson to record Hey Jude.
The Wilson Pickett recording of Hey Jude is how Clapton first heard Duane Allman's slide playing and caused him to phone the record company to find out who it was.
This led to Tom Dowd introducing Eric and Duane.
Which led to them jamming together.
This led to Duane being a part of the Layla sessions.
Layla cemented Duane's reputation as one of, if not the, greatest slide players who ever lived.

Such a serendipitous string of events which had such a profound effect on modern rock as we know it.

[Edited on 9/28/2017 by BirdsAway]