There are many indirect risks, but the extent of the risk is pretty much unknowable. Maybe the vast majority of these viruses are degraded and no longer viable. Perhaps some were obsoleted by the evolution of more competitive relatives or stronger host immune systems. Perhaps the vectors and hosts are extinct and the virus has no means to naturally replicate. Maybe the one virus that manages to breakout is enough to do serious damage to the environment and society. Or maybe there are thousands of different catastrophes waiting to thaw from the ice.