About 3-4 years ago there was a study showing that the half-life of DNA was much shorter... like around 500 years.

https://scitechdaily.com/researchers-calculate-that-dna-has-...

However, I've always wondered if it would be possible to reassemble the whole DNA via aligning overlapping segments by indexing MASSIVE amounts of DNA.

IT would be VERY expensive to do now with today's tech but might be posssible in 10-20 years.

It should be, but it might not work with contemporary RNA in host cells for cloning.

They have dozens of Neanderthal DNA going back to 70K years. (They stopped being a distinct sub-species 30K years ago) In humans the skull next to the ear are good place to find intact DNA.