Speaking for myself, I know that a lot of the reason why I have been able to adapt to the 6s Plus is entirely psychological. When I first saw 6 Plus last year, I thought it was monstrous, but I was still intrigued by the large screen and the beautiful build (a first for a giant phone when it came out). Eventually, I found myself desiring the phone more and more, and as that happened, it started to seem much more manageable to me.
Basically, I wanted to get used to it because I liked it so much. Now that I have the 6s Plus, sure, it's not as easy to hold as a 4.7"-screened Moto X--few phones are--but I've found a way to make it work for me. I don't want to use a smaller-screened phone anymore, so my desire and willingness to get used to the 6s Plus tips the scales in its favor whenever I encounter any awkwardness, which is increasingly rare now.