According to the specs, the supercaps may also fail within 10 to 20 years due to lifetime issues. They do not last forever and have a significantly lower lifetime than normal caps.

I've actually opted for batteries for very small currents (µA range) because the battery lifetime is about the same as supercap lifetime, so why bother with a charging circuit?

Two batteries would be nice. Or a battery plus a (normal) cap backup that lasts a week or so (or at least the time it takes to change the battery).