TL:DR
I was wondering if a spare battery with separate charger, case, and a spare backcover is really a benefit.
A separate charger that needs redesigned with every change of battery form factor doesen’t seem to be a sustainable product.
In addition to this it is only usable for this only usecase.
That would be even more parts to lug around.
Imo a powerbank is a more universal device for the job. Once you have it, it charges almost all mobile devices, is usable for many years and doesen’t wear out the FP backcover.
So in total it may be the more sustainbale and universal choice to provide many devices with the needed juice.