It's hard for me to imagine wanting to use a phone for anything other than making calls or sending SMS; that's what I've been doing for many years now and I see no reason to change. But if I did have a tablet or laptop, I could just sync the program to it and run it locally. Maybe using, for example, good old rsync.
And I can't imagine being away from "home base" on a laptop for long enough (or using it for anything critical enough) to really worry about how to achieve "centralized backups". I'd rather not transmit that data over the Internet when I could just connect the laptop physically to my backup storage when I got home.
Not even listening to music in the car? That is probably my #1 phone use case by far outside of the communications functions you mentioned. I run a Navidrome server at home, and while on the go my phone can stream any music I like from the home server (so I don't need to load it in advance). In theory one might store the music on one's phone of course, but I have more music than my phone will hold and it's very nice to be able to access whatever I want to listen to at will.
But aside from that, inside any metro area even a 200ms or so buffer would likely suffice for streaming music.
200ms and even n+1 won't cut it for a subway, a semi-basement pub, a tunnel, a train or an airplane trip, a hike, a countryside visit, etc.
I generally prefer to have access to all my senses on public transit, but there are any number of other portable devices I could use that store the music locally.
> I have more music than my phone will hold and it's very nice to be able to access whatever I want to listen to at will.
If I were going to choose from among that much music I might as well search the Internet anyway. An entry-level microSD the size of my thumbnail now holds a couple hundred CDs worth (at uncompressed CD quality; several times that for high-quality opus).