Well, they were literally forced into it. Can you imagine the tech media and consumer response if they attempted to release another 4" phone. They'd be taken (and laughed) to bits. Funny thing is that this applies to the 4 inch, but the reported 5.5 inch, that I feel was just a choice. I get having to compete with the likes of the Note 4 purely for commercial reasons but that really is the kludge bit, completely unnecessary. Although maybe they'll target it towards designers, artists, and professionals like architects. Screen real estate to display images, create, or use high end software like CAD.
There's nothing stopping Apple launching three sizes with modern hardware.