The 6 Plus has not replaced my iPad. One of the big reasons is that I got cellular with my iPad for this gen.
Also, I do a lot of PDF markups for my academic work and it is easier on my iPad.
I have switched over to reading more Kindle books on my phone, but usually because the phone is always with me, while the iPad is put away at night or for short trips anywhere.
I do use the phone more often for web browsing, which surprised me, because I thought it would be too small, but it is just the right size for casual browsing. For anything more extended, though, I grab the iPad.
The iPad is just too necessary for my academic work for my phone to replace it. Still play graphics intensive games on the iPad, too.
My phone gets more use now, and my iPad a little less for some tasks, but I don't see either replacing the other anytime soon. I actually wish the 6 Plus were a little larger, but even then it would not replace my iPad.