I hear this mythical date in 2140 when we find the last scrap.. but does it have to be.. if only we could keep halving.. for-EVER!
You'll be adding an ever smaller number of Bitcoins per block. There's no point increasing the total Bitcoin supply by - say - 0.00000000000001% per block. The limited supply is by design, and it's one of the things that makes Bitcoin better than fiat.
The M2M economy, as I mentioned earlier, works orders of magnitude lower down the real-time trough. They'll be chatting away hundreds of times a second, querying each other for data etc.. the 'yearly' spend per device will be less than $5.. chopped up millions of times. There are 5 billion Sim/WiFi enabled IoT devices launched a year as of today. Trillions to come. These numbers get really big.
Can you give an example of a device that would need to send and receive very small payments hundreds of times per second? There's probably a better way than adding money to that protocol. The only thing I can think of, is something like a webserver, and even then a small advertisement makes much more sense than nanopayments.
Can you fork every time, maybe every 20 years.. add 8 decimal places ?
I think you're underestimating the magnitude of 8 decimal places. There's no need for 100 million times more of the smallest units.