This balancing act is already happening. If you modify the battery controller, you can totally continue charging beyond the voltage that the phone considers to be 100%. It also increases the risk of damaging the battery (https://www.acebattery.com/blogs/what-will-happen-when-a-lit...). What they define as 100% is already some point on a damage probability curve, and charging to anything below that point will further decrease the amount of battery stress (for li-ion batteries and similar technologies)

Fwiw, based on tests I've seen recently such as https://www.youtube.com/watch?v=Lj4LMlGr4og, I think limiting to 80% is overblown, but somewhere in the 90%s could be a sweet spot that gives you several hours' longer battery life than with 80% but still has a much reduced chance of significant degradation. I don't understand why they didn't make this configurable