20-Year Prospect
We don't have the best outlook on this one. Sorry about that.
The thing is, automation means that within a few decades many jobs will cease to exist as we currently know them. With regard to recycling, we may be looking at heavy automation in the near future (source).
For one thing, self-driving cars are rapidly becoming a reality. All of those trucks that pick up recycling will soon be driven around by ghosts obsessed with empty soda cans. If you don't need drivers, you don't need someone supervising them.
The same goes with the sorters on the line. The more that process can be automated, the better. It exposes fewer people to danger and eliminates human error. Machines don't get motion sickness from looking at belts, and they don't mind bad smells.
In twenty years, this job is going to be less of a supervisory position and more of a tech support situation. It will be about keeping the machines up and running while they chew through the garbage, some of which will be recycled into other machines. However, someone will still have to be responsible for making sure the plant runs smoothly, so it stands to reason there'll still be MRF managers. We just can't say for certain what the job will entail.