Alec in his own video mentions that the issue isn't that the Americium decays, but that the electronics themselves age and fail, which applies to both the ionization detectors and photoelectric detectors.

This is one of the things you just don't wanna mess with, as such a failure is completely unpredictable, and from what I know some manufacturers are even beginning to make detector units with non-replaceable batteries, intended to be replaced whole when the battery dies after years of runtime, to make it impossible to keep using a detector after its rated lifetime.