You’ve drilled down to the critical issue - batteries. For better or worse, we have observed there’s a limit as to how much stuff people will willingly carry on the regular. Current phones barely fit in that space envelope. Make a phone twice as big or require a separate device, and for most users this is equivalent to not carrying a cellphone.

I don’t think the current dominance of the big two can end until the hardware and software requirements of making a good phone are much much more accessible to normal developers/engineers than they are today.