My 6 plus isn't close to lasting all day. And I have everything turned off, push, transparency reduce motion. Etc etc etc.
I wonder if your usage is as heavy as mine. For my usage a 4000 mah bat would be ideal and I'd give up thinness for that in a heartbeat.
I have no idea what you are doing with your 6 Plus then. I use my fairly heavily with the following turned on - BT, WiFi, Personal Hotspot, Push and background refresh. Still able to get through the day and most of the next morning before having to charge again.
If you aren't happy with it, take it back. Or, buy an external battery. Then you can recharge whenever you want to.