I doubt this effect will last long, maybe 4 months. China is already rebooting its economy, including power stations, factories, etc.
Regarding the reduction of the world population, the virus is striking most of all where population growth is basically too low, anyway. And we in the Northern Hemisphere will find ways to survive this except for a couple thousand mostly old people, who are not exactly the biggest polluters, anyway.
In short, I am not overly optimistic regarding the positive effect of corona. And don't expect humans to become reasonable and humble just because of such a crisis. Humans are still like those idiots on the Easter Islands who ruined their environment until they died out.
The major positive effect so far is that the Eurovision song contest has been canceled