My own experience of photography is different from that described by Sontag. That is not to say, however, that I don't recognize those "momento mori" aspects of what I do. I would, however, say that it is more true that all photographs are a celebration of being alive. To take a photograph is to see (or an attempt to see) something immortal, invulnerable, and immutable.
I take photographs because something inside of me says "There it is." [What it is I am never always clear.] It is about the moment. It is not about the moment after that or before that. All photographs are affirmations, from this perspective, of the particular moment of being alive. Each photograph from every photographer says "This is what it is to be alive for me."
Before they remind us of death, they first affirm life.
Sontag would have known this if she were a photographer.