Lithium batteries are last century's technology
They're quite competent in powering ordinary wrist watches, children's toys and our motherboards, but smartphones require something much better.
This is especially more pertinent when we are at the cusp of VR/AR technology becoming mainstream. When streaming videos and video chats are becoming more common. 8 hours or one day of use just wouldn't cut it anymore.
Some people say we are slaves to smartphones. I say we are greater slaves to charging their batteries, keeping a vigilant eye on the battery percentage indicator, coming up with feeble methods to compensate for the constraint: quick charge, wireless charging, power-saving mode, power banks, charging stations in public places etc.
Not too long ago Nokia 2G candybar phones could last an entire month on standby with a lithium-cadmium battery. Somehow, during the transition to smartphones, we're conditioned to accept the drastically reduced battery life as the new normal.