The original bipolar variant of NE555 is likely to have a lifetime of many decades, if not more than a hundred years, even when operated continuously in harsh environments.

A modern CMOS microcontroller has a much more limited lifetime. Depending on model, you can hope for 10 years or 20 years, but not much more than that because very small MOS transistors and flash memory cells eventually die, unlike the more robust bipolar ICs (whose active regions are buried in the semiconductor crystal, not located at its surface, like in MOS devices).