Temporal Limits on What Engineers Can Plan

My question is: How far into the future is it possible for engineers as such to plan? For example, the Yucca Mountain Nuclear Waste Repository was to have been designed to store nuclear waste safely for between ten thousand and one million years. Is that the sort of planning engineers as such can do? The planning engineers do would not be philosophically interesting were it not in general so often successful, much more successful than the gambles of ordinary life. So, how is such planning possible-and what are its limits. Is one million years beyond the limits of what engineers, as such, can plan? Is a thousand years? Is a hundred years? Is there an nth generation for what engineers can plan? The answer I consider here is that engineers can plan only as far into the future as they can reasonably expect engineers to be present. That is only a few generations at most.