One issue is that you need a bunch of space behind the focal plane in a digital camera (the thickness of the sensor + its mounting system + PCB + cooling gap) while film cameras can mount the film (~zero thickness) right against the back door of the body. Another is that people expect EVF now, which requires more envelope volume than a compact 35mm OVF. Generally people paying $$ for a camera expect it to have removable lenses which adds again to the physical envelope.
Theres lots of super thin digital cameras, an inch thin or less (coolpix, cybershot, etc). An iphone, which has a better screen, more capability and cpu power than a camera is extremely thin, what 'cooling gap'? Are you saying that the thickness of the sensor goes up as sensor size increases?
And theres plenty of $$ cameras with no removable lens, fuji x100 series, Sony RX1, leica Q, etc, like you linked.
There have been some Sony camera with fixed 35mm focal length lens and FF sensor. Like this: https://www.bhphotovideo.com/c/product/1316682-REG/sony_cybe...