It's true that the batteries don't last 6 years, but they are perfectly user-replaceable for someone with moderate technical skills and a few basic tools. Here's a kit that has everything you need:

https://www.ifixit.com/Store/iPhone/iPhone-6s-Battery/IF314-...

Otherwise, Apple will replace the battery for $49 while you wait:

https://support.apple.com/iphone/repair/service/battery-powe...