[lutusp]

> When our species had existed for around 5 years, does that mean it was reasonable to assume that within another 5 years, we will have been replaced by another species?

You're missing the point that our species didn't arrive that quickly. From the perspective of 199,995 years ago, assuming a certain amount of intelligence, our forebears would make a similar assessment -- that they had assumed their present form some hundreds of thousands of years ago. And we have copious fossil records to support the idea of a typical duration for individual species before they disappear.

In any case, it doesn't matter which numbers one chooses within wide limits -- the fact is that our species will disappear, no matter what we do, within, say, 500,000 years at the most -- meaning our descendants will not be recognizable to use, could not interbreed with us, and would not be obvious kindred spirits to us.

We're a transient species, and we're not in charge, nature is.