I don't think this setup is all that different from the real world. You will need activists to move people, just like before. I find it rather promising that there are organizations forming with specific purposes that can potentially change the behavior of others on this platform. Steem cleaners, for one.
Maybe you are assuming too much about what a successful platform looks like. For example, from a purely advertising point of view, it matters more that there are enough eyeballs on the platform, no matter what trash is there. If steem continues to attract and retain users, this could be what it looks like. Contests run by advertisers and trash flying around, and some thoughtful gems here and there. I'm actually quite surprised in its current state the variety of content that is thrown out there, and even creative ways of leveraging this platform.
That said, it's not perfect, but the good part about this kind of system is that it can evolve. Whatever types of tweaks made to incentives, we at least got a lot of minds scratching our heads about what the possibilities are. Maybe the right solution is throwing it all out and redesigning too, certainly a possibility... For now, this has proved to be quite an interesting experiment to watch.
I'm sure if Steem gets no competition, and continues with its roadmap (whatever that may be) it will be successful. I like looking more at ideal solutions or robust solutions rather than ones which are fragile to certain elements of human nature. Granted every solution will be fragile to some element of human nature, but those that are more robust tend to survive and thrive.
I'm sure Steem that will evolve, but will it evolve too slowly? But I guess that's part of the experiment. The Steem developers do have interesting ideas, but they are kind of slow at implementing them (ie Hivemind, Communities)