Actually, it was the X that did that. It was the best selling iPhone in the lineup when it was released, which really makes it the "mainstream" phone, even if it doesn't really fit the definition.
But, I get where you're coming from.
Still, phones are kind of small and we can only cram so much tech into them (although I would argue that they are the most tech-dense items in our existence). But, really, it gets down to, "what else can a phone do?" What it looks like is only part of the equation and, really, the least important (unless your phone's fashion statement is more important than what the phone can do for you). Seriously, what else can the things do? They have replaced our, well, phones, our walkman, our camera, our tour guides, our GPS, our mailboxes, our social skills … well,
The things are everywhere and do a ton of things. What else do you want them to do? The bezels are gone, they're thin, they get pretty darned good battery life, they're fast, they have eleventeen cameras, and stereo speakers…
Sheesh!
AR? VR? They're on our wrists, even! They can't really get smaller (although many would buy that), and any bigger and it's an iPad. How fast does it
really need to be? There's only a 1.5" x 1/4" space left to cover up with pixels. I'm seriously trying to think of anything else that cannot be handled in software (which is why I don't get paid to sit in a dark room and brainstorm the future).
What's left? Folding phones? Yeah, that worked out. Yes, they're in the future, but innovation for the sake of innovation serves only to embarrass the innovator. Accordion phones that fold out into 24" x 12" scrolls of digital over-indulgence?
Man, modern phones are cool as it gets. Gonna take a lot to make them even more cool!