I wonder if that was the real trick of the iPhone: slightly less than a day is enough battery; work on that basis. It was dropped into a world where battery life was measured in days.
Before mobile broadband, there would not have been as much reason and application, but with mobile broadband, the entire game changed, and Apple smartly had ATT give it unlimited so people did not have to worry about using it.
Slight correction, I had to go look: AT&T started rolling out UMTS (3G) in 2002. I already had a 3G smartphone in 2004 in Europe that could do video calls.
Fun fact: UMTS was announced in 1999 with a mind-bending 384kbps downlink speed.
And with the advances in technology, most people can get slightly more than a day which means the phone gets charged overnight. And that’s not really an inconvenience at all.
Anything less would be a burden, for most people roughly a day is good enough.