While this is astute and true, big apparel is also about scale and logistics. Shoes, in particular, are notoriously difficult to get into. UA f'd up because they got too big for their britches. I was on their site the other day and it looks like they're trying to be everything to everyone. That's not really reasonable for a business of their age in their market.
Look at Nike, as an example. They got into the golf market and established themselves as a major player, but are still a very small player in the extreme sports market, which they've been trying to get into for at least 20 years. Besides golf clubs and balls, is Nike equipment really the first place you'd go for a basketball, soccer ball, football, backpack, baseball bat?