Battery life is an area that may be difficult for smaller phone makers to compete on. I think Apple especially puts a ton of engineering effort and coordination into making iOS and their apps work efficiently with their hardware, reducing complexity, runtime cycles, and power consumption as much as possible, on top of already highly-efficient ARM hardware.
Over years of doing that (kaizen), the result is optimized hardware/software fusion with industry-leading battery life. But it seems like it takes a non-trivial amount of additional engineering time and effort to accomplish this, that will be difficult to match by smaller mobile tech startups.
I hope the open source community around Librem and Pine will be able to replicate that effort, but I'm not sure this kind of consistent incremental upgrade work is attractive enough to volunteer FOSS developers. And being maximally effective at it most certainly requires the parent company to coordinate the effort across hardware, software, internal teams, and external volunteers.
I’ve read somewhere recently that in order to keep the phone mostly free of proprietary firmware, Purism had no choice but pick lots of discrete components. That would be in contrast to most other smartphone designs with more closely integrated chipsets, they wrote.
That discrete-ness, according to the author, is likely to be an upper bound for battery lifetime on the Librem 5. After all, all those chips have to be powered at least part of the time, and that allegedly consumes more energy than a single package would.
In other words: the Librem 5 may never gain a decent battery lifetime. As drivers mature, battery usage may improve a little. But not much. Buyers may want to keep their hopes realistic.