It may surprise you, but a part of design is fashion [trends].

My Ski jacket from about 10 years ago still works really well. At the time it looked great, but now it's outdated in style, although it still works great.

So as a designer, yes I worry about trends, because if I don't then you'll end up picking a jacket that works and is on trend over a jacket that just works (other things being equal like price etc).

Design is not a single input discipline. Many factors go into making a design for a product... including trends and fads.