That is all relative to your knowledge and usage of course.
Yes, that adds some overhead for the smallest things, but in that regard, if all you want to do is to light a led, then I2C is overhead compared to a simple GPIO...
About the cost, a simple STM32F042 which can do USB crystal less is less than 3$ by single unit from digikey, and it can play the role of several I2C components (like IO expander, ADC, DAC, PWM UART at the same time...) which would add a lot of overhead if done through several I2C components. Again, it depends on what you want to use it to.

May I ask what kind of usage you have in mind ? (Remembering that is for a "relatively costly" phone, and not a "relatively cheap" arduino or raspberry pi that you can bury in anything you want and forget about it)

So we have to agree to disagree here, as I would be for USB first class (as it can do ALL I think about), and if that really doesn't cost more to add pogo-pins (which I think is not the case), then add all you can, like I2C. SPI without I2C is not always useful, as a lot of SPI devices need I2C for configuration (like the lepton camera).

We'll see what Chen thinks of this, as he usually have great ideas regarding what is possible and cost-effective to produce.

I actually found they did a good work on this, as can be seen in the PDF from the archive downloadable there: https://sailfishos.org/develop/the-o...developer-kit/
The website has been updated since then, but the docs are the originals from February 2014.