> Li-ion battery arrangements inside those devices seem to hold about 80% of their usable capacity and voltage for about six months, with average usage patterns. A year after that this drops to 50% of original capacity, or below. This is known, if a bit too generalized.

What? Taking the example of Apple (since they make their battery ratings easily accessible, but other manufacturers are similar), iphone batteries are rated to hit 80% at 500 charge cycles, and ipad/macbook batteries are rated to 1000 cycles [1]. Assuming you discharge your battery from 100% to 0% every day (which is at the upper end of average usage), iphone batteries should last ~1.5 years, and macbook batteries around 3 years.

Apple also has a pretty good battery replacement service, at least in comparison to other manufacturers. When my iphone battery wore out after ~3 years, I shipped it to Apple and paid $60, and they sent it back a few days later with a new battery. My macbook battery lasted ~4 years of heavy usage, and they did the same thing for ~$200. A user-swappable battery would definitely be easier (and cheaper), but the service they offer is good enough that it's still feasible to get a replacement and keep the device past the battery lifespan.

[1] https://www.apple.com/batteries/service-and-recycling/