After all, in the life of a keyboard it probably types 10,000 characters per day for 50 years. Call that 400 Million events that need to be sent to your mac via bluetooth low energy.
The system can be entirely powered down when no key is pressed, so the only energy loss is a pre-keypress amount. That works out to about 3 Watt-hours (assuming each keypress is transmitted 3 times for interference-resistance and has a packet length the same as an advertisement). A long-life alkaline battery has a low enough leakage to last 50 years, and about the volume of an AA cell can easily power daily use for 50 years. If you want to go smaller and lighter, you could get 10 years out of a coin cell.
It would probably work out cheaper because you don't need to ship the device with a cable or charge circuitry too. Users don't have one more thing to worry about charging either.
