I feel like you are overly simplifying that. Battery’s degrade based on charge cycles, and yes charging to only 80-90% of capacity certainly helps prolong life. Driving 10k miles a year vs. 100k just means more charge cycles and a lowered capacity (all other things being equal). They also just degrade over time. I haven’t owned anything li-ion powered for more than a decade, at least not yet. Tablets and phones batteries certainly don’t last 20 years. Obviously charging habits and capabilities are different, but I wouldn’t think it’ll be the norm to have original batteries in a 20 year old EV.
As others have mentioned - serviceability is the main issue. Force IEEE or SAE standards on packs, cells, or modules. Make rebuilds a reasonably priced option. Things fail in engines, not all are catastrophic. A stuck lifter isn’t expensive for parts and cost far more in labor. However a lot of home mechanics could replace a lifter. Even before insurance gets involved, few people are going to tear into a sealed pack to replace a cell.
