Buying a phone mainly for a camera is silly. Nobody wants a bad camera on their phone, but 'deal-breaker' is a bit extreme — it's not like they're terrible now.
I imagine if it's so important to you, you can't have used an iPhone earlier than the 4.
I, like many other people upgrade our phones ONLY FOR THE IMPROVED CAMERA.
I don't really care about any of the other features (although things like improved processor speeds are welcome).
I'm a dedicated iPhone photographer and the iPhone is the only camera I use. Period.
I prefer the 6 Plus size screen as I also need a large DVF (digital video finder) for shooting and the screen is much better for editing photos.
So a 'high-end' iPhone with large screen and dedicated photo/video capabilities is what I'm interested in.
If you think about the size - an iPhone 6 Plus is still smaller than most compact cameras and certainly smaller than a DSLR. Arguably the battery lasts way longer too!
I have been on the fence about purchasing a dedicated camera - i.e. mirrorless Fuji, Sony, Olympus etc or a DSLR (Nikon, Canon etc) but I love the workflow and portability of the iPhone.
Also as the technology improves, the gap closes between what a phone can do vs. a dedicated camera.
Sure, a dedicated DSLR is more flexible in many ways than a phone - I'm not getting into technical arguments here.
But if the next iPhone considerably "ups" the photography capabilities then that gap gets even smaller.
Obviously megapixels (MP) are one thing.
A high megapixel number doesn't mean much when combined with average optics.
There are many phones out there offering 20MP+ images that actually don't look that great (even compared to 8MP stills from an iPhone 6 Plus for example).
Also, WHO takes the photos is as important that WHAT they are taken on.
So as the lines blur, with the launch of the iPhone 7 and better camera optics etc - I'm holding off considering a 'dedicated' camera.
Ideally the iPhone 7 will have 16-20MP, better optics, integrated multi-camera lenses to create ultra-sharp images with much less 'noise', image smearing etc and the ability to make larger prints without much post.