> millions of which will last a century with proper care

I strongly doubt that. Electronics are not designed to last that long; components age. Also, planned obsolescence is king these days.

How are you going to charge those devices reliably and consistently? What knowledge-bases will you be able to connect to with no Internet?

Our technology stack is like a house of cards. Smartphones are at the top. There's no way they will be of any use in the event that the foundation and the walls collapse.

There is a great short story with the unfortunate name "The man who came early". It's about a American GI who is stationed at Keflavik during the cold war and somehow gets thrown back in time to the Icelandic saga age. He wants to give people there so kind of technology that's much simpler than a smart phone and it all fails. Even something simple as gun powder doesn't get off the ground. They don't know how to get Salpeter. Even if the had the powder they would be missing so much technology to make the guns. As the main character says "you haven't the tools to make the tools to make the tools". Also resources.