"ultra low" ? Isn't that quite high ? it would deplete a 2000mAh in less than a day while idle ? My Jolla with network active can stay for at least 3 days on a single charge with minimal use.

That's the solution used by all (most?) integrated GPU even on Intel processors and some external ones (like my Nvidia GPU on my desktop). There are cost and complexity issues that lead to these choices.
It also gives flexibility, if you don't need the GPU memory (running headless for example), you can take it back for the CPU...

No idea...only interface available maybe ?
Or they were targeting some small sensors and not "real" extensibility of the phone. At some point i remember Marc Dillon talking of things like blood glucose sensor for diabetes, which would work correctly with I2C.

Really ?
A bunch of them exists : https://www.verizonwireless.com/accessories/moto-mods/
And except for keyboard and battery, none would have been possible with I2C.
All the TOH where hacked by community. The infrared imaging would not be as useful (8x8 pixels), compared to the normal ones you get from FLIR (works easily on all phones with USB OTG, so a huge market compared to the TOH) are at least 80x60 pixels with fast refresh, that for example gives enough resolution to allow troubleshooting PCBs with detail on with SMD component is dissipating more than it should.
But we already had that talk a lot of times in TMO... and were are again out of topic