Another thought -- what about the Battle of Jutland? OTL, it was pretty inconclusive, but did the Royal Navy lose its chance to crush the German naval defenses?
I don't think so. Really it achieved all it was going to considering it was more successful than originally planned by Brusilov.
Yes - the Somme was never going to be anything but a disaster for the Entente, but Brusilov very nearly knocked Austria-Hungary out of the war and would have succeeded in this regard had Evert launched his part of the offensive...
I'm by no means well read on this part of the war, but FWIR, the battle is, at most generous, described as a "learning experience" for the Allies. So even if, for example, Haig had realized that bombing the barbed wire wasn't enough to remove it as an obstacle, the offensive would still have been catastrophic?
It's ironic that the campaign that, OTL, went much better than expected would be the one with more potential, but, as of the moment, it looks like that's the case. Actually, the even bigger irony is, currently, it appears the Brusilov Offensive is also the PoD
with the most potential for the CP to win the war in 1916.