Woah, decrease in usability? Let's slow down there a little bit.
There's a legitimate case for big phones, in that they make typing somewhat easier. Typing on my iphone is pretty annoying to me, so I'm actually happy that a 5 inch size is coming to be more standardized.
Not to mention the impracticality of web browsing on a small screen. My Galaxy S3's screen is still smaller than I'd like for web browsing, but it's usable which is a hell of a lot better than my iPhone.
