Rapidly deteriorating battery life has long been tolerated by smartphone owners, mainly because they have no choice. Phones usually meet the holy grail of battery life, the ability to last all day, at first. Then it starts getting worse quickly, until many phone owners end up plugging in during the day. It seems it's always been this way, and there's no incentive to the smartphone makers to make this better.
Battery has long been the focus of the mobile segment. In the early days of mobile the possible time away from a power outlet was not very long. This prompted OEMs to do what they could to make batteries in mobile devices last longer.
This is why we now have decent battery life with most laptops and tablets. The eight hours needed to go all day no matter what comes up is now met by many mobile devices. Except the smartphone.
Pick any smartphone in the top ranks and talk to those who've owned one for a while. Odds are, you'll hear a familiar tale — the phone easily lasted all day when new but that didn't last. If they've owned the phone over a year you'll probably hear about the antics they go through daily to have a phone that is still working at day's end.