Imagine a mobile phone that:

has the power draw of today's chips, but using the old speed (I dunno, 32 mhz?) - same performance, lower power draw, smaller size

has the same battery size as today's phones (more effective battery than the old ones)

has an e-ink display (much better battery life; pretty sure even the passive matrix displays or whatever they were called were less efficient than today's e-ink)

uses only what is needed for calls in terms of radio (no drain from connecting to gps, wifi, 5g, etc. Just regular gsm)

Screen size still tiny (unless there's barely any power draw difference between a 1 inch screen and 4 inch screen on an e-ink)

I dunno if digitizers take a lot of energy, but physical keyboard would be awesome

Anyway, point is, I'd imagine this would be like crazy effective. Even throwing a contemporary battery into an old Nokia would probably make it last like, I dunno, 5x longer. And that's just one change.