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.
It has existed since years ago. Gekkoscience do have a USB miner that outputs 20+ GH or so.
You can also be quite certain that big players could design faster/smaller/more efficient chips.
Why would they? If consumers do not want it, they wouldn't think of implementing it. It really seems like you are underestimating the additional cost and benefit of companies actually doing this. Its really pointless to put a single chip into a device.
It looks to me like completely decentralized mining is within reach. Laptops consuming 50 watts for example could easily accomodate 5 watts overhead.
Alright fine. Then how about the cooling? How about the cost of companies actually purchasing chips to stuff it into their device which 50% of the users wouldn't use?
It's not within reach. Even if you were to implement chips in the device which every Bitcoin user is using, it would hardly match the current hashpower that most farms have. Furthermore, the hashpower will not be constant, it will fluctuate ALOT and Bitcoin will experience slow block times at certain time of the day. Anyway, it is rather hard for the protocol to be changed such that only those who mine can use Bitcoin.