The BM1387 ASIC chip is capable of ca 0.07 TH/s and consumes roughly 5 watts.
You would need some 70 millions of those installed to match the current total network hash rate. No need for billions or users.
Honestly, I don't know why you have refused to listen to my comments on this. No phone or laptop will ever be able to support this. The power consumption is way too high. Less than an hour of mining and your battery will be drained.
Phone and PC/laptop manufacturers could add an ASIC to their boards for example.
Or dongles could be made (possibly combining OTP generation with mining killing two birds with one stone...).
Quite honestly the "price" to pay (ie device + energy costs) is well worth it !
What do you think ASIC chips are? LEGOS? Do you seriously think that anyone can smack a big ASIC chip into a device with a LOW TDP design in mind? The price for the design could very well cost more than several HUNDRED thousands and the phone would each cost more than $100 extra and not everyone would use it. Honestly, no one would buy it.
I believe that the major issue would be rather on the network bandwidth / latency side of things.
I do not know if bitcoin has been designed with such a heavily decentralized mining in mind.
Its not. Why the hell would it be? Decentralised mining will NEVER be realistic.
I gave you an example of a good commercial existing chip to explain that there is no need for billions of users.
That chip is using 14nm technology (and is less than 20mm2 btw), so by far not the most optimized thing that could be achieved with the most recent technologies.
We are not far from the 2.5watts of USB2.0 for example, which opens the door for simple mining using USB dongles.
You can also be quite certain that big players could design faster/smaller/more efficient chips.
It looks to me like completely decentralized mining is within reach. Laptops consuming 50 watts for example could easily accomodate 5 watts overhead.
So, the question is more about the network related issues, and the underlying bitcoin mining protocol requirements.