20-Year Prospect
Here's some good news: Fewer students are being diagnosed with learning disabilities every year. Here's the bad news: The number of autistic students continues to rise (source). This is most likely due to the increasingly sophisticated ways we can diagnose autism rather than anything in the environment causing it. In any case, the point is that a special education teacher is going to have different challenges than their predecessors in twenty years.
Autism is a good example of this. The way that this condition is being diagnosed and treated is constantly changing.
Currently, the trend is toward "mainstreaming" the kids. This isn't some attempt to make the hipster students hate them. "I liked Braden before he was big." No, this is about putting the kids with social, physical, or learning disabilities in the same classroom as everyone else (source). The preliminary data looks pretty good, with reading scores rising across the board.
What this means is that every teacher is going to have to get some special education training. As a result, fewer dedicated special education teachers will be needed. The position will usually be reserved for the cases that can't be mainstreamed, while general special education techniques get wider use by all teachers in general. If you're dead set on helping special needs kids—and no need to polish that halo, it's shining plenty brightly—just be ready to be doing it along with other children who don't need so much help.