20-Year Prospect
Unless humanity starts to embrace its more animalistic instinct and decides to forgo clothing (caveman-style), fashion itself ain't going anywhere. That doesn't necessarily do you any favors, though. Trends come and go within a matter of weeks, and what is in today is going to be considered outdated in five years, ridiculous in ten, vintage in fifteen, and suddenly totally in again at the end of two decades. You're just going to have to ride that wave and try not to fall overboard.
Globalism is another issue that American fashion has to come to terms with. After all, most of the clothing Americans wear is made somewhere that isn't America. And since half of all clothing manufacturing jobs have gone overseas in the past decade, this means there are fewer positions to go around for those of us who really want to make it here in the good old U. S. of A. (source).
You can definitely last in the industry for twenty years, but you'll have to be very lucky to remain on the top of your game for that long.