I think this reader has just about the best defense of the ending, if not necessarily of the finale as a whole, that I’ve gotten so far today.

I think people who want the “everything goes black” ending to signify Tony getting whacked are “overreaching.” For one thing the scene is not exactly shot from Tony’s perspective.
I’d say the non-ending does provide closure: the core of the show has always been about the psyches of Tony and his immediate family. And, at the end, Tony has come to terms with all of them (even apparently his father and Uncle Junior).
Tony’s therapy has in fact ended successfully from his point of view (or as badly as Melfi came to fear): his un-self-aware, sociopathic self, has been validated.
There is also closure in the completeness of Tony’s corruption of Carmella and the kids. There was always some doubt about Carm’s complicity in the mafia life (e.g. her complete naivety regarding Adrianna’s fate); by the end there is no doubt that she knows exactly what Tony is and that she is along for the ride.
The screwed-up, suicidal, angst-ridden AJ is bought off with a BMW, some nookie with an underage fashion model, and an apprenticeship with Little Carmine. So much for his worries about how screwed-up the world is.
And even talented, idealistic Meadow has found herself: she’ll marry a mobster’s kid, and pursue a big money career as a mob defense lawyer. Even better, she has acquired Tony’s capacity for blind self-justification: she’s really doing it all in the name of civil rights.
The narrative may have broken off, but the psychological journey of the characters has been completed – what more do you really need to know about any of them?