I didn't say "it failed because it didn't do X". They said "... failed to gain developer adoption ....". To that specific point, I contend there is a clear and direct causal link between the decisions they made, and that lack of adoption. Of course I can't prove that in the strictest sense, but it's not hard to see that a lot of (potential) G+ developers kept asking and asking and asking for usable APIs and never got them. Speaking only for myself, as somebody who was initially a G+ fan and might have been inclined to build things on top of G+, I never did so for exactly this reason. Yes,"n=1" and all that, but a lot of other devs were very public with their position on this as well.

Obviously their decision to close it down involved multiple factors, but the lack of developer adoption was specifically called out by Google as one of those factors.