How many preorder their iPhone before even testing it? These people don't care that much if it is 3,5" 4" 4.7" or 5.5", they just want to have the new, one and only premium iPhone.
By offering two premium models, they will have to choose whether they want more display+better batterylife or one thats fits better in the pocket.
They may buy an new iPhone and still feel not having the nonplusultra iPhone.
If Apple is about to bring an Phablet, they will have to rename it to iPhone Air/Mate/Note... and bring it with a delay. Just like Samsung with its Note, LG with its Pro and HTC with its MAX.
Sounds stupid but people are stupid. People will also get upset if manufacturers update their phone too early, even it wouldn't make their own phone worse.