With the family tablet yes, my phone hell no!
"Do you think that's air you're breathing?" ;-)
What I dislike most is handing someone "my phone" for them to use for a minute and they are actually using my phone. If they authenticated and were immediately interacting with their phone I have no problem sharing the hardware for a minute when the wife/kids want to do something briefly on it.
I understand the point you're making though. You want a phone that's a terminal to the cloud, with caching. That's a fine use case I suppose. Wouldn't stand in your way.
Many phones are cheap enough today (not to mention hand-me-downs) that there isn't a huge need however.
But it's also bigger than that. If everything you do user-land can be synced down fairly instantly to any piece of hardware - phone, tablet, desktop, TV, watch, etc. - it provides a level of mobility and usability which can enable some very powerful use cases.
Approaching a device and have it immediately be "yours" is important for the screens in the self-driving ride you hail, or the shared workspace you might rent by the minute, or even the TV you sit down in front of in your own living room.
But this could even extend to the POS terminal which you checkout with at a store, a screen you walk up to in a mall, a digital assistant you approach in a store, an ATM, etc.
FaceID is transparent walk-up/pick-up authentication, which is table stakes for some very cool possibilities.
Your kids want their own device. They will get it sooner or later. Mobile devices by nature are personal objects, complete with personal greasy screens and battery levels we have nobody else to blame for.
Approaching a device and having it become "yours" might be a fine idea for certain applications, but I'd argue the living room TV is better off having a default profile which everyone in the house uses. If one person's profile is lagging or missing content or apps or settings, they fall behind and we now have a frustrating scenario of some profiles better than others for watching TV. Obviously sub-usage areas such as Netflix makes sense to have different profiles, but not the whole TV.