I really don't like when people use the term 'the perfect size'. 'Perfect size' is something that is completely subjective to the person using it. Obviously the 6+ is not the 'perfect size' for my 4'9'' aunt.
I agree that the 4 inch phone needs to stick around. Sure a nicer screen gives you the ability to view more content, to have less eye strain, etc., but some people really just don't have a desire for these things and Apple would be ignoring such a large portion of the market that desires these 4'' phones.
Apple has 'laser focus' on their product line, which means they don't like supporting so many different variations of products. This is unfortunate for people desiring 4'' phones because history would tell you they're not coming back. I just think a 4'' phone makes too much sense for Apple to ignore, however, and while it may even be 2016 when it occurs, I think we'll be seeing the 4'' phone once again from Apple. Even if it is a budget model or something along those lines.
Me personally, I enjoy the size of the 6 and I'm torn between keeping it or even getting a 6+ for the extra real estate and battery life. I can realistically see myself getting any of the 3 next go round, so it'll be interesting to see where I stand in a few months time.