Realistically the next real leap in phones will probably only come if and when battery tech improves

I don't think so. The real thing is just ergonomics. They want as big a screen as they can and human hands are only so big. They won't retreat in form factor.

If anything they'll are toying w bit with folding screens to make devices scale up and still be hand friendly.

The next big step is going to have to be wearable, to produce arbitrarily large content hands free. Don't know if social norms will tolerate it, but that's really the possible evolution from the status quo.