20-Year Prospect
While people have been crying about the "death" of the theater industry since the advent of HDTVs, people still seem to attend public screenings in droves. We have no real reason to think that will change. People like being shown to their seats. It's likely always going to be a part of fancy events.
The position of usher has undergone some changes in the last twenty years (especially as the movie theater experience has become more informal), but remember: an usher wears many hats. As janitor/ticket taker/candy seller/security, you'll always have something to do no matter what the era.
It just may end up being less...usher-y.