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.

You can make the phone 5 times thicker though, I remember the late 90s and it was still carried everywhere

That's what Purism did with Librem 5. Wifi and cellular modem are both removable.

You could essentially do just that using a small tablet or other device (Raspberry Pi?) and VOIP calling through a WiFi hotspot.