You completely nailed it. First you say that no one is inventing posteriori arguments, and then you go on to say that "changes can have effects". But, but non-changes can also cost human lives in the long run. However, you CHOOSE to defend non-changes under the (in my view, irrational) belief that the changes might cause loss of lives, even though you are perfectly content with the consequences of the non-changes, which might also cause loss of lives in the long run. Example: "if we had fewer deserts, we would have more arable land, and so food would be cheaper, and so fewer lives would be lost". So we have one argument that says that changing might cause loss of lives and another one that says that not changing might cause loss of lives. Why do you always pick the non-changing? (when I say "you" in the previous phrase, I'm referring to people who call themselves environmentalists, who always make the argument that change might cause loss of lives, and never the opposite, not you personally).
Desertification can cost us anywhere between 25 to over 50% of the global arable land over the next 50-75 years.
The destruction of natural habitats can also have a negative effect on human lives, if only by denying future generation from being able to visit them. We gain nothing from destroying them in the long run, many of them even if they are deserts can be natural barriers to desertification, and again there is no dichotomy in this case I'm really having hard time trying to uncover any argument you might be attempting to make.
Simple question: do you concede that in some cases keeping the natural habitats as they are might be worse than changing them, in terms of loss of life over the long run? If I can't get a yes or no on this, I don't think there's much point in continuing the conversation.
No, I'm not trolling.
Yes by not destroying some natural habitat and causing the white tailed ferret to go extinct humans might bring their own extinction because in 3048 they are going to become the natural carriers of the Z virus bringing the oh so anticipated zombie apocalypse.
But meantime in the real world from what we know so far from both research and experience negatively affecting the environment causes negative effects on the human civilization.
Nature tends to balance itself out, this is slightly harder to do with humans which have the technology to geoengineer their habitats.
But even if it was so it doesn't matter, no person in their right mind would think it's ok for me to die, and for millions or billions of other people to die just because it's the natural order of things.
And for the most part you can't even hide behind it's a "natural process" because humans are geoengineers we build huge concrete cities, we build dams, we divert rivers, we strip whole mountains and dig the decomposing biomass of our "ancestors" and burn it as fuel.
Bears and trees don't do that.
Desertification is a big problem and it has many human linked causes, large cities change rainfall patterns, deforestation causes soil migration changes the mineral content of the soil and reduces its moisture content and most importantly desertification affects humans as it creeps into arable land increasing the cost of growing crops or in some areas making it almost impossible.
The other problem is the destruction of natural deserts due to buildup of man made structures, agriculture, diversion of water sources, overall climate change (a desert doesn't have to be a "hot" place, many deserts are actually in danger due to rising temperatures which increase rainfall, melt glaciers, or soil locked ice) and many other things.
No one is inventing posteriori arguments and decides arbitrary what is good and what is bad, one environmental change can drastically affect human civilization including costing human lives in the long run, the other is unnecessary destruction or damage to existing natural habitats which do not have much (negative) impact over human civilization or lives but can and should be prevented if not reverted. This is not different than any other preservation effort, you want bears in the wild, doesn't mean you want them living in the woods just by your house, and it also doesn't mean that you would not shoot them if they come and endanger human lives.