I really doubt that. Never underestimate mankind's ability to mess up.
But even if you are correct, there are a lot of cosmic things that can eradicate our little civilization or even species when it is confined to a single planet.

If mankind stays bound to a single planet then it is ultimately doomed.

It's demise might come quickly due to global warming, nuclear war, a global plague or a million other causes we are not even thinking about right now.
Looking at longer timeframes there is the danger of a cosmic impact, natural changes in the environment (ice age,...),...
And even if we deal with all these we will still be gone in about 10^9 years because the sun is slowly moving towards the end of its lifecycle.

By moving to other planets and having them become self-sufficient we can deal with a lot of these issues and extend the lifespan of the human species (or its evolutionary descendants) with millions or even billions of years.

And once we have become a truly interstellar civilization (or even an intergalactic one) we can start playing the long game and prepare to deal with the eventual heath death of the universe; extracting energy from black holes, building outposts around dwarf stars,...

A dedicated civilization with proper long term vision could potentially last for many billions of years ones it has a truly interstellar domain under its control.
But that would require leaders who think beyond the next series of elections...

Suppose you found pieces a c t g… How could you know if the correct order is actg, atcg, agct,… ? It would be virtually impossible.

Thousands of years ago is easy.
Changes are high that you'll have a lot of their descendants living today.
By observing them, noting the differences between de descendants and the earlier versions and making comparisons to other animals alive today you can make excellent educated guessed.

Further more, you might have other types of evidence such as footprints, human drawings of the animal, descriptions,...

And finally, you might even be able to get DNA samples or find partially preserved remains.

If we're talking about animals that lived millions of years ago then we still use the same principles but we just don't have the same abundance of data.
But we could for example extrapolate that a dinosaur and a Casuarius that share similar legs and pelvic design would have walked in a similar fashion. If we then find fossilized imprints of the dinosaur's feet and compare them with those of a Casuarius (stride length, spacing,...) then we might use that to confirm or correct our assumption.

This is of course a very brief description about how we can formulate rational assumptions by combining multiple pieces of information; both ancient and modern. So, just understand that it's much more complex then a brief explanation can do justice.