That's nonsense. I bought an iPhone 6 Plus the day they came out and realized in about 5 minutes flat that it sucked as a tablet. The aspect ratio was wrong, web browsing in landscape sucked, gaming sucked, (*sucked compared to a mini) pretty much all tablet specific features sucked.
The mini is absolutely a great device with so many size specific use cases, whether it's portability for commuters, a pocketable tablet, perfect size for drone pilots to mount on the drone remote, aircraft pilots to put on a kneeboard or yolk mount, in bed reading / viewing, on and on and on.
Yeah any one of these use cases is "niche", but when you see comment after comment with people all saying the same thing: "I use the mini for X because anything bigger is too big and anything smaller sucks".
That’s because you bought a phablet not a tablet. I think the Plus phones are fantastic! In fact I find myself going to my iPad less and less since I bought the 7 Plus. The only thing I don’t like is that it is almost too big for my pocket. Hopefully the X Plus will solve this problem.