There is a major difference between the two projects: we can tell whether a manned mission to the moon has succeeded or failed. There is no way to tell whether the effort you put into making a clock last for 10,000 years had any value; you cannot observe the results unless the clock fails incredibly quickly.
A lot of the mechanisms and designs involved in the clock will be testable from Day 1, for instance the effectiveness of the energy harvesting system, which is designed to power the clock from the beginning without external power inputs.
Additionally, things like corrosion and wear resistance performance will be measurable after just a few years of operation.
There is an entire field of engineering, Reliability Science, based on the fact that we can predict the lifetime of a system based on a combination of accelerated tests and detailed physical models.
