I think you could attach a USB-hub to increase USBs (?) for ethernet there is probably also a solution
Perhaps one could. But then one has a sprawling mess of parts and cables and wall warts not a nice small simple all in one solution.
For the operation systems it seems just a matter of effort, the same effort as had been done for the pi should have worked.
Yep. When are you going to start putting the effort in?
I think a big issue here is that is not just one tablet/mobile hardware platform. They are all different, they are changing all the time. Who wants to put all the effort in keeping up with all that. The Pi is the Pi, for years now and years to come. Much more efficient use of manpower to support that.
modifying existing platines should be easier than creating your own ones
I presume you mean the printed circuit boards (PCBs). Might be. If one had the design files. Which we never do. And could get hold of the SoC and other parts easily.
I think OPs and community could easily be added to an android-phone or tablet
I presume "OPs" means Operating Systems. Not so easy. We look forward to seeing what you come up with.
You cannot "easily add" a community to anything. People gather around what is interesting and useful to them. You cannot buy a community to order.
...just the platine with the chips looks superior and smaller in my new tablet.
How so exactly. The Pi PCB is of very high quality. The chips look great to me.
They connect the display, cameras with a special small flat-cable.
There could be an adapter : flat-cable--> IO-pins
Such an adapter is another complication. Something to make it harder to work with. It adds expense. It add unreliability.
The lipo, speaker are soldered, an exchangeable connection could be added
How is that a good idea?
...the community could select special types and agree on standards
and build additions and software around it ..
What community? See above.
Chinese wheels are produced in larger numbers and cheaper,better.
Not so. The "wheel" that is the Pi was originally made only in China. Now they are made in the United Kingdom (Wales). For the same cost and quality.
It seems to me that those could be adapted for your education task instead of creating entirely new platines
Perhaps. Again, we look forward to seeing what you come up with.
Sorry, but I'm finding it impossible to follow your logic.
You seem to have a plan to adapt cheap Chinese mobile platforms into something that the Pi is already. This is a huge project you have set yourself. The end result will not be cheaper.
Why not just get on with creating interesting projects using what is already available, the Pi.? It's not as if it was expensive now is it.
Memory in C++ is a leaky abstraction .