Librem 5, PinePhone and Shift phones, to name a few devices/manufacturers currently working on such ideas.
there is no rush to kill play store or apple store. ms could relaunch a windows phone but ms do not want to waste in consumer hardware, it's better to them invest on other types of "OS's" like github, azure, xboxlive, which are also virtual markets. the question that remain, can Intel make a x86 phone? I guess it depends energy research.
all-in-one is a brand. there is no real all-in-one.
Nothing. It's just very very hard.
One issue is that the mobile form factor doesn't lend itself to laptop or desktop class cooling. Some mobile chips can approach the performance of a desktop... until thermal throttling.
There is a reason there are different kinds of cars and trucks for different market segments. Try to build a car-truck and you get the El Camino. Computers are likely going to be subject to similar market forces.