If I recall correctly from my reading of Endgame, he thinks that the natural course of civilization has an apocalyptic endgame--he doesn't specify exactly how it will come about, but its cause will be the consumptive, exploitative nature of civilization. Therefore, civilization will do itself in. He also feels that the more we speed that process up by revolting against civilization, the better for the natural world (and for us, too--as we ARE a part of the natural world).
He would like to see all this happen before we wind up destroying everything. Nature will rebound, but it would be better to not have to start from square one.
Here's one of the Premises of his book:
Premise Nine: Although there will clearly some day be far fewer humans than there are at present, there are many ways this reduction in population could occur (or be achieved, depending on the passivity or activity with which we choose to approach this transformation). Some of these ways would be characterized by extreme violence and privation: nuclear armageddon, for example, would reduce both population and consumption, yet do so horrifically; the same would be true for a continuation of overshoot, followed by crash. Other ways could be characterized by less violence. Given the current levels of violence by this culture against both humans and the natural world, however, it’s not possible to speak of reductions in population and consumption that do not involve violence and privation, not because the reductions themselves would necessarily involve violence, but because violence and privation have become the default. Yet some ways of reducing population and consumption, while still violent, would consist of decreasing the current levels of violence required, and caused by, the (often forced) movement of resources from the poor to the rich, and would of course be marked by a reduction in current violence against the natural world. Personally and collectively we may be able to both reduce the amount and soften the character of violence that occurs during this ongoing and perhaps longterm shift. Or we may not. But this much is certain: if we do not approach it actively—if we do not talk about our predicament and what we are going to do about it—the violence will almost undoubtedly be far more severe, the privation more extreme.