This post doesn't give any actual argument why you should charge your device only to 80%. If you use your phone for, say, four years, and charge it to 100% regularly, how much will the battery life deteriorate during those four years? I assume if the fully-charged capacity after three years is still at least 80% of the original, then regularly charging it fully was absolutely worth it. The point is that deterioration in battery capacity has to be compared to the loss of usable capacity if you only charge it to 80% voluntarily.

By the way, they also warn from fully discharging the battery, and recommend an charge range from 30% to 80%. This would mean you limit yourself to only 50% of the capacity. It is questionable whether gains in battery lifetime can outweigh this loss in realistic scenarios.

The main lesson seems to be that you shouldn't fully discharge your battery when you can avoid it easily, and not fully charge it if you know in advance that you won't need the full charge.