I like the latest phone each year and new tech. It's cheaper to buy phones than it is to have two year contracts.What it is for you is no relevance for overpricing. If 800£ is ok for a phone 800£ for a much more powerful device can't logically be overpriced. It may not fit your usage case but that has nothing to do with a fair pricing.
And why are you changing gear yearly? That's insane. I plan for at least 4 years for phones and the iPP will probably hold around 4-6 years before I need a newer model.
For me both mini and air are redundant. For the go I have a iPhone 6+ and for all other mobile usage the iPP.
It's overpriced when you compare it to an air 2 for the average persons usage. It's faster, better speakers and has pencil support so really is that worth an extra £400? Not for me. Not like the air 2 is slow either.
Pro is a fantastic device and is great for most people who have one