Cargo remains more profitable than people even today. That's why you can still ship packages via passenger bus. Or at least you could the past time I did it in the early 2000's via Greyhound Package Express.
I'm not saying it Amazon Airlines should look like United, with a couple of hundred people up top and all the cargo down below. But maybe add 50 or 60 seats in a space in front.
One of the problems with cargo is that it's so ephemeral. A plane on a particular route can be full of packages at Christmas, and then half empty the rest of the year. This is exactly the sort of problem that Amazon seems good at solving.
Maybe make the passenger compartment modular. Add more seat modules when cargo loads are low. Add more cargo pods when demand is high.
If anyone can make it work, it's Amazon.