There could be anything underneath all the permafrost, including new biodiversity which we haven't catalogued yet. NPR ran a story[0] where Russian animals such as reindeer infected with Anthrax had thawed out, and caused outbreaks where people had to get preventative treatments against it. As a future threat model, what if there's new viruses or bacterium contained under previously frozen earth for which modern medi…
Such events are necessary to keep human population under a manageable number. Throughout human history there have been outbreaks that claimed many lives and stunted population growth for some time. We will likely see such an event in our time.
Your thesis other than being fictional also is an emotional appeal to the concept that what is natural is desirable which is wholly unsound both on its face and because any die out sufficient to be effective is entirely likely to not only take civization with it but also reduce our ability to take effective measures to manage our environment as our technology dies with our civilaztion.
The way forward to a managable population size is through raising everyone up to a relaively modern level and the way forward to that population living sustainably is via education.
Edit: Your philosophy that its ok for the more developed nations to wreck the environment leading to mass death presumably among those you regard as your inferiors while relax and have a latte safe behind our metaphorical walls is pure insanity.