Biologists have DNA samples of every known species.

I do not believe that be true. Even if it is, a single sample is insufficient for a meaningful statistical analysis.

I believe it's false. Good-sized animals are still being discovered. The ecology of micro-organisms is still being explored.

Ok, but how much value would it place on an organism which wasn't adapted to the modelers environment, as demonstrated by the fact that it was selected against and went extinct?

Non-negligible, depending on the criteria. It was my belief that human caused environmental destruction was the issue at hand. The organism was adapted for human's natural environment (most of Earth), the environment changed.

A lot of what's worth finding out at our present level of knowledge isn't about whole organisms, it's about specific aspects-- consider the work being done with spider silk. Spider silk would probably still be valuable even if there weren't any living spiders.