I'd say the majority of smartphone users already have a solution for that... Anyone who has ever used their smartphone for navigation bought a $5 car charger, and will be able to charge up their phone, and their neighbor's, for at least a few days.
Personally, I've got a pocket-sized $10, 1W solar panel that'll charge AAA/AA batteries, and/out output to USB, so I'll be good to go indefinitely (as will anyone else who has ever gone camping) and I'll keep my immediate neighbors up and connected, too.
And I will reply that you're wrong, as the majority of the people in this world don't own a car, and by the way things are going, fewer and fewer people will manage to get the resources (and the desire) to buy one.
So, no. Relying on automotive chargers are suicide on the long run.
Solar chargers will help. for sure. But only on sunny days - and emergencies don't happens only on clear, bright, warm and sunny days,
the analog phone operators can redirect their power supplies in order to keep the phone lines working
POTS has problems with being overloaded after disasters, too. Authorities used-to encourage people to look for pay phones and hang-up the handset if they've fallen off to help... POTS is superior, but it benefits from its' declining use, in the same way cell towers suffer from their popularity. Cell towers, like POTS, could be designed with more call capacity. Government agencies like the FCC simply have not set a high enough bar for cellular service, to ensure available in emergencies, like they formers did with POTS.
*Every* service will have problems with overloading after disasters. Every single one of them.
What I'm advocating is that different services have different characteristics, and so have different resilience to different kinds of emergencies.
I think that there're situations where POTS will manage to save more lives than cell phones, in special, long term blackouts where a powered plug will be something that you simply will not have access.
Of course there're situations where cell phones would do better - what is a good reason to have both!