An argument considering the different outcomes of two extinction scenarios
It is impossible to say if human extinction would be a tragedy without knowing the circumstances under which we were wiped out.
Not all extinctions have the same results. Consider two scenarios. One where we die due to a pandemic disease which wipes out all humans. One in which we interbreed with space aliens creating a new species.  In the first instance, there would be no humans left to wreak the environment, preserve our works of art and literature, or continue to further the understanding of the universe and our place in it. In the second instance, humankind might be extinct, but the new dominant species could inflict more damage to the environment. In which case, human extinction might be a tragedy. Alternatively, the new species of half-human, half-alien may have increased reasoning and understanding, in which case some may argue that the loss of humankind would not be a tragedy. Without all the details it is impossible to make a comment on human extinction.
The circumstances don’t matter. The very loss of the continuation of our species would be a tragedy. The only marker that we were ever here is the continued existence of our species. It is the only imprint left on the earth from our evolutionary forefathers. If humans became extinct, it would be a tragedy for everyone that ever came before us. It would erase their legacy. In as much as we have an obligation to protect the interests of the future generations, we also have an obligation to protect the legacy of those who went previously.
P1: Some circumstances which would cause human extinction would be a tragedy. Some would not. P2: Without knowing the circumstances, it is impossible to make a sweeping judgement.
P1: Regardless of the scenario, the very end of the continuation of our species would be a tragedy.