Yes, the iphone will get near 600dpi in the next few years, competition will dictate that.
One could certainly see in 3 years 600dpi being the default for all devices outside of large monitors or tv screens. It's already being worked on and these new screens use way less energy, prototypes are already out.
2048 x 1536 was selected because its a multiple of the old iPad, and the next few years it will most likely stay the same. Right about the time 4k tv gets rolling, it will go higher, probably double if it doesn't completely change form factor.
But the next revolution in dpi will take place with desktop and laptop displays since most are under 110dpi and look like crap next to an iPad.
You can't rule out a new technology not even invented yet taking the lead in a few years. The ultimate outcome of research going on now will be:
In the not so distant future, screens will look better than paper in the best art magazine.
600-1000dpi, (same as some professional art print)
No glare, Illumination only when needed.
Paper thin
Very little power consumption.
QLEDS are looking very interesting, very low power and can be up to 1000dpi with way less power consumption.
We look at screens all the time, why wouldn't anyone want to perfect them to be as close to real life as possible? People have said in the past, do we need color? Or a million colors is overkill, nobody needs over a gig of ram....
People will say how did we ever live with such low dpi? Just like we say about memory, color, monitor size etc...