If I run my Samsung Note 4 in it's extreme power saving mode, it's got a standby that is something like 17 days! I can easily go a few days, if I'm just talking on the phone, text, and the occasional web browser session.
If I want to play audio, it will do that for the better part of a week in that same mode. Just point Chrome at the files and go.
Old phones didn't have apps that required constant connectivity and didn't have screens at all, essentially.
So the problem is that this is comparing yesterday's phone with today's pocket-sized computer.
Note: you can still buy a feature phone today if you'd like.