I'm sure Google could monetise Gmail more than they have, but I've always thought the real business success of Gmail was sharing its login with google.com and moving Google to a position where many searches are done with a personal identity that they can track anywhere. It seems before Gmail there wouldn't be much reason to actually log in to Google, and having users logged in makes their product better and I'm guessing gets them way better data.
I think that Youtube was more successful in that space (for regular people at least). I base this entirely on the fact that around the time of G+, the redirect urls for all Google products started going to YT, which I took to mean that YT had a much broader cookie pool.
FWIW, I agree entirely with your point, but I think Youtube is the secret sauce that makes it work.
