I really don't think coach seats are getting narrower. Boeing's narrow body fleet all have the same cross section that was originally designed for the 707 in the late 1950s. While many airliens originally operated these aircraft in a five abreast layout, the change to six abreast happened decades ago. Since then, there has been no reason to make the seats more narrow. Newer planes, like the 777 actually have wider seats, upwards of 18 inches, in coach.

People are getting wider, seats aren't getting narrower.

Legroom is, of course, a different story.