20-Year Prospect
As long as music and music performers aren't going anywhere, neither are band managers. According to the Bureau of Labor Statistics, the number of agents and business managers for performers and athletes are expected to grow by 9.7% by 2022—about the same pace as other jobs (source).
So keep searching for the right band to pair with, and keep working to get them (and yourself) to the top of the music world.
The other thing that needs to stay the same for band managers to keep their jobs is the hard-partying personas that rockers are usually trying to embody. It's possible that all of the sudden rockers could become much tamer—going to bed early and scrupulously managing all their expenses, bookings, and behavior on the road—but it's unlikely that will be the case.
In truth, many musicians seeking fame will probably also still be seeking a wild lifestyle in twenty years, so they'll need someone like you around to keep things under control.