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But you'd have to make the hundreds of broken connections between the boards you split. So isn't that a really delicate ribbon cable?

Not really, you do the split according to functionality, not just a straight split. CPU/memory on one side, WiFi/BT/Baseband on the other.

So something like Google's project Aria that they just cancelled with separate modules for each function. Only not user serviceable.

Like any phone. They all have separate components and boards connected with ribbon cables. You can see the iPhone 6+'s various ones here:

The proposal is just to do a little more of that.