Given the rate of climate change and how little we're doing, trying to make 10000 year institutions seems foolish. We can't even manage the institutions we have now.
It's not about planning out 10,000 years, or even 100; it's something more abstract than that. What will your legacy be? Your family's legacy? Your country's? Humanity's? I don't have any personal connection to somebody who will be alive 1000 years from now, or even 200, but I still don't want to leave them a dead, depleted planet.
Is the clock a bit frivolous? Sure. But it's not about the clock. The clock is merely a symbol, that we have considered our place in history, that we recognize we are part of something larger, and we want to be good stewards of our future.