OP, I want to see a video of you quick replying to a message that drops down from the top of the screen or quick marking an email as read that also drops down from the screen.
Also, accessing the Notification Center from the lock screen and also pushing the sleep button on/off but while still being able to access the Home button one handed.
You mean like the drop down notes from notification center? I mean, obviously it would be impossible to reach them without using the gesture, but it wasn't exactly easy to do even on the 5/5s. Generally if I saw the notification, I'd just tap on the messages or email app in the dock at the bottom and reply that way.
But yes, you've got me there, if I absolutely needed to do the quick reply to a notification drop-down it would likely be next to impossible, as would accessing the notification center itself.
As for the sleep button, I doubt anyone would be able to push the sleep button with one hand while still being able to reach the home button unless they are Andre the Giant or JJ Watt. That said, was this something you could do on the iPhone 5, or even the previous 3.5" iPhones? Honest question, I don't have my old one here in front of me so I can't test it, but it seems like it'd be a tough thing to do even on those models, especially since the sleep button is up on the top of the device.
For me, the real crux of the issue isn't "Can I do everything one-handed", but is instead, "Can I do
most things one-handed?" For the most part, I'd have to say yes. It is perfectly functional to use one-handed in most every day tasks. I personally will still use two hands as often as possible because that's just always how I've used my phones since smart phones became a thing, but in a pinch, one-handed use is not a big deal.