there’s likely problems with draining the external battery to the point that they’ll never consider it.
1) Right now, "shorten lifespan" mode
already exists anyway, it's the "use extender first to point of heavy taper"
2) I advocate for changing
that mode only to start proportionally charging from main battery at point right before the taper occurs, and delivery is always smooth, no taper occurs to mixed charge due to temporary dead zone. This would in NO way worsen that mode's impact on lifespan, so your argument on this incorrect.
Do you understand what adaptivity would do ? It would effectively switch from mode1 (extender only) to mode2 automatically, without user input, which is what everyone right now does.
But you had to build a strawman.
Surprise, this is already how other systems are doing it. Are other engineers dumber than Spec? No, Spec is just lazy to do feature updates.
Re Battery: 2000 KM on my KSL this year (maybe not enough, but I have toddler and spent equal KMs on pumptracks), almost always discharge to less than 10perc, then fully charge to 100, leave for days if must. Battery still at 100perc. health. Baby-sitting lithium batteries has become obssesive. Impossible to do any long-term damage to battery on gravity oriented bike, no one on KSL or LS runs 20K per year.