The design of such ASIC could be open sourced - removing the barrier of entry, ensuring a level playing field.
Hence, removing the competition for ASIC manufacturers. Open sourced does not mean that they will follow that design in their ASIC.
Then, every device on the planet can easily be fitted with these ASICs and the overhead of doing would be negligible for HW manufacturer.
Uhh. No. There is definitely some overhead to this. It isn't possible for you to fit an ASIC chip into your phone together with your CPU. The hashpower is negligible too.
I do not think it would be necessary to reward miner in a heavily decentralized scenario (say, > 100M miners) - the fact that you would get secure transactions for the cost of the energy you spend seems to be enough of incentive.
No rewards=Lesser people having incentive to mine = easily attacked. You cannot assume that there would be 100Million miners at any time. If you can fit it into a phone, the will be very insignificant due to the low TDP design.
Then, I also believe that any user node should (must) be a mining node.
That is, it should not be possible to use the system without contributing to mining.
No No No. If every node IS a mining node, everyone would have to fork out their resources to the network. If I have a phone and I want to use Bitcoin, do I have to risk overheating my phone just to use it? The idea is frankly impossible.