Well with hindsight maybe the GGG flight did damage Brook more than we first thought. But don't forget, these guys are trying to make as much money as they possibly can . Brook would've neede 2, maybe 3 unification fights to earn the money he did against GGG .
Who's to say he wouldn't have got beat by Spence anyway?(and he probably would've got beat) and then the earning potential goes down.
It could be the end for Brook (with the exception of a fight I don't want to see happen with Khan,) but his earnings potential couldn't have been surpassed.