I can also go several weeks off grid with literally any phone, a moderately sized power bank and a 40w solar panel hanging off my pack or over a tent without thinking too much. It's far more versatile for powering other devices and I never have to reboot my phone. If you want to carefully buy stuff you can even get a fully IP65 rated or better setup, which makes it actually survivable to the elements.

I can't see how juggling internal batteries is anything but the worst possible option. I can upgrade or replace any one component without obsoleting the rest. How many future phones will accept your stockpile of batteries?