I am disappointed there's no single player story mode etc., if I play SoT it'd probably be solo because of my lack of free time.
I agree about the single-player mode, but one thing I will say about the lack of skill leveling, etc., is that it allows groups of friends go get together and play. I play co-op games with a couple of friends spread across the U.S. and one of the three of us rarely finds time to play...maybe once per week for a few hours. The other two of us have a lot more time to play, so if this was a progression heavy game, it would just make it more difficult for us to hop in and pirate around for a few hours a week. But as such, I can see this being something we play in smallish doses for quite some time, especially if Rare adds things over time.
I'm totally aware that's not the experience everyone is looking for, of course. For us, it's basically something different to do on game nights instead of Gears or Overwatch.
EDIT: Also, I'll admit that Rare could have kept this just as friendly for infrequent players or casual groups while adding things like oh, enemy variety instead of just 10,000 skeletons and an occasional stronger skeleton. This game is really polished in terms of graphics, mechanics, etc., but definitely lacking in overall content and variety, from what I've seen and heard.