The premise of this seems excessively optimistic -- and/or excessively egoistic. It's plausible that humans will still exist on Earth 8,000 years from now, but that requires fantastic optimism (as in it requires fantasy to predict not only the future you can foresee/imagine in your likely lifetime, but for many, many millennia and lifetimes beyond your own). On the other hand, assuming humans (in whatever new form) are inhabiting this earth are still here, starting to timestamp your works (writings, etc) with 5 digit years says you really expect your outputs to be in existence then. Though they very well might be, does one think that without your foresight to use 5 digit years it would maybe somehow be lost, or difficult to archive?

Unix's time limit is based on the capacity of 32- or 64-bit integers (hardware). For denoting years, we lost nothing because the ancients didn't write dates as 4 character years -- we, and that info, survived adding digits.