People should probably just stop hoarding so much stuff that they (think they) need a walk-in closet, but even if that doesn't happen there are so many better, more versatile ways to store things. Like an armoire, for example.
I would bet money that in not very long, walk-in closets will be way out of vogue, much as city planners now look down on the surreal tract housing neighborhoods that produced the American cul du sacs. For now at least, walk-in closets seem to be a still-popular vestige of the ignominious McMansion.
If I ever remodel my home, I'm not only going to avoid installing walk-in closets, I'm going to rip out the existing small closets. I will only use armoires, and my house will be all the better for it.