I think it would be a nice thing to include as not everyone wants to interact with other players if they don't need to. But I don't think it should be an obligation.
It sounds like a concept that I could enjoy, but I probably won't play it if there isn't a PvE mode. I play video games so I don't have to interact with people. But I'm not going to buy it knowing there isn't a PvE mode and then complain that there isn't one.