I think that 'closure' doesn't really apply to the Sopranos. A conventional ending would have been disappointing. I don't think it was done purely as a cynical cop-out or to get people talking. I think that, to a certain extent, the Sopranos had its own path all along and it was going to pursue it regardless of viewers' opinions.
I'm not saying that David Chase did it solely because of artistic conviction... he obviously knew that the ending was going to be the focus of much discussion, and cement the legacy of his show. Still, I don't think he did it just to screw with us.
As for Tony being dead... I think it makes more sense.
I haven't read the analysis yet because I want to watch the final episode again to see if I come up with any new insights, but here's what I think so far:
- The song playing at the end is by Journey. The band name and lyrics ("he took the midnight train going anywhere"

in this context seem to allude to death. That said, I realise that the song content could be used to argue the opposite ("it goes on and on and on and on..."

- The people in the diner seem sketchy (although this strikes me as a deliberate attempt to distract us from something essential)
- The guy seemingly re-enacting the Godfather bathroom scene
- Tony's earlier conversation with Bobby "I bet you don't even hear it coming"