The back-up batteries in Canon's no-back-up-battery cameras is rechargeable and pulls its juice from the main battery. The only time it gets discharged is if you leave the main battery out/dead for extended periods of time. Since it is usually fully charged, it goes through very-very few charge/discharge cycles.
For example, my 60D is five years old. It has never been without a battery for longer than it takes me to change batteries or install/uninstall a grip. As such, it has never undergone a charge/discharge cycle. I expect my grandson will be getting this camera in 5 or 6 years. I also expect the backup battery to still be working fine when he throws it out because the shutter has finally failed.
Barring a mechanical problem, I would expect this design to work for quite some time. Well past the time when Canon stops servicing that particular body.


