These points exactly. For a while I loved the huge screen and the landscape mode. But I can think of maybe three apps that have really made use of the extra screen space. Most developers haven't done anything special for plus users. Also the split screen view in the default apps hasn't proven as useful as I'd hoped due to the small screen size compared to an iPad. Also, the landscape keyboard on the plus is atrocious ergonomically. Now that the newness of a having an iPhone phablet has worn off, I realized I still reach for my iPad whenever I really want a big screen and mainly use the plus the same as all my previous iPhones. Except the plus doesn't always fit nicely in my pockets...
The only thing I would really miss about the plus is the OIS. The sharper screen is definitely nice, but I don't notice a huge difference over the regular 6 unless I am holding the phone really close to my face. But OIS has allowed me to get some really nice low light shots off my 6 plus that simply weren't possible on my 6. I am definitely disappointed Apple didn't add OIS to the regular 6S, as that would have made the decision to switch to the smaller phone WAY easier. At the end of the day though, both are just smartphone cameras that are limited by a small aperture, plastic lenses, and no optical zoom. And, if I'm really honest with myself, the vast majority of the pictures I take get texted to family and friends or posted on social media. Either phone will be just fine for those tasks.
I think I'll be switching from the 6+ to the regular 6S this time around. At least I'm 90% sure...