since the next year Apple will introduce an iPhone 6.5" model...with probably ipad UI...the ipad mini is dead
This is incorrect.
I have a Samsung Galaxy Note 8 (6.3") and an iPad Mini (7.9"). I can assure that the screen real state difference is HUGE, plus the apps and websites render on tablet mode, not phone mode (which is also huge difference). Furthermore, the screen ratio on an iPhone (18:9) is different than the iPad (4:3)
All those factors make the iPad Mini tremendously more comfortable to use for content consumption than my Note 8, despite the theoretical difference being only 1.6".
Screen real estate depends on more factors than just diagonal inch size. That is precisely the reason why the iPhone 8 Plus screen (5.5") has more real estate than the iPhone X (5.8") despite the theoretical smaller screen.
In short: the 2018 iPhone X Plus will be no substitute of any iPad, Mini or any other.