Part of the reason we've had conditions replace the ones we've eliminated is that, well, basically -- things that previously would've killed you don't now. It's a fair bet that this is part of the reason autism is on the rise: they think that a lot of children that died of 'failure to thrive' were in fact autistic. In the modern age, they are able to live long enough for their disabilities to manifest.

So -- I don't know. I tend to think that we'll keep beating things and keep improving quality of life, and that once we make certain breakthroughs (most particularly as relates to stem cells and genetic therapies), disability may become increasingly rare as we find ways to cure what would only have been treatable at best in the current era. (Which is actually perhaps horrible to contemplate when we consider that in it becomes rare it's possible that people will start getting worse in how they treat people with disabilities.)

But then again, that said? I suspect that if we fling ourselves to the stars, we'll find new and horrible things out there to keep having to outmatch. So I doubt major medical conditions will ever completely disappear; it'll be a never-ending game of cat-and-mouse, as it were.