Beyond just battery flash slowly wears out over time, degrading performance. Based on my Nexus 6 I would love it if the EU dictated batteries must be replaceable, but you need an overabundance of flash so a few years in there are still cells left to balance wear across.

The Nexus 6 automatically throttled performance based on battery left, but at some point the battery wore out to the point that less than 1/2 an hour of use got you below that threshold. After that the phone was very laggy and frustrating to use. No way anyone would want 5 years of that experience, updates or not.