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That defeats the point of having a big battery. You're sacificing battery life to gain battery longevity

Yes and no. A battery's capacity isn't the only variable. You also have internal resistance. If you have a worn out to hell battery where there's dendrites on the anode and you've basically electroplated the cathode the internal resistance is going to be much higher. So if you draw the same current on a worn battery and a new battery the voltage drop is going to be way higher on that worn battery. If you're already at a low state of charge and you vdroop your way below 3.2V under the current load that battery is going to cut the current. Most people notice it as an Android phone that randomly reboots at low battery.

I've bought a phone with a big battery so I could do exactly what the GP said, always charge between 30-80%. But that means I'm cutting half of the potential energy in the battery.

I'm lamenting the fact that we have to do this even though film capacitors can be drained from 100% to 0% without damaging the capacitor or reducing its capacity. This is far closer to how you want an energy storage device to operate. Aside from the fact that film capacitors store about a thousand times less energy by every metric.

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