I have the 6s+, and yeah I can't use it with one hand, but I'm okay with that because I love having the bigger screen so much more. I can see a lot more while typing, watching videos is a better experience, and I've noticed myself using the bigger phone (I used to have a 5s) in new ways a lot more.
Before, I would avoid doing some tasks on my phone like taking more than a few lines of notes. The bigger screen feels less cramped and I don't feel annoyed while using it. In fact I wrote this comment out on my phone. I've never done that with any of the smaller phones I've had.
I think it probably has to do with the kind of lifestyle you have. For me, I have to commute by bus, and most of the time it's packed so I have to stand. While on the bus I'd like to use my phone to avoid boredom, which means using it with one hand because I need the other to hold onto something.
I can totally see how having a bigger screen and bigger battery are great. But unfortunately for some of us, we have times where we need to be able to use the phone with one hand.
You're right - there's no one size fits all solution. I'm glad that phones have gotten bigger because they fit my use cases, and based on their popularity I'm definitely not alone. I am surprised that 4" phones have been lost - especially by Apple. Despite the low sales of the 5c, I figured we'd still see a 6c to satisfy those not wanting a bigger phone. It seems like a decent number of people are refusing to upgrade to a newer iPhone, and I'm not aware of any good, modern Android phones at 4" for these people to switch to.
The closest you get to a good, small Android is Sony's Xperia Z(1,3,5) Compact series. They're about 1 cm shorter than an iPhone 6/6S mainly because they don't have the home button, but that's still about 0.5 cm taller and 1 cm wider than a 4" iPhone.
My use of the ipad has definitely descreased since I have gotten the 6+. That extra effort of getting up, locating where the iPad is usually isn't worth it for me since the phone is within reach already.
Before, I would avoid doing some tasks on my phone like taking more than a few lines of notes. The bigger screen feels less cramped and I don't feel annoyed while using it. In fact I wrote this comment out on my phone. I've never done that with any of the smaller phones I've had.