"Astronomical" isn't an actual number. You still have to decide if it's a billion dollars or a trillion dollars or a billion trillion dollars. It's possible, and harmful, to choose a number which is too large.
I find myself agreeing with you and at the same time thinking that your position is absurd. Let me try to put this into words. I might be missing parts of your point.
If we measure out the cost of avoiding total environmental collapse - which also implies human extinction - and we put a finite number on it, we've said 'the survival of human life on earth is worth this much and not more'.
It's really hard for me to get my head around the idea that we would find a dollar value that has any meaning that would properly describe the limit of how much humans value their own survival. I wouldn't know where to start, what to measure, or what to exclude. I'd personally be loath to excluding anything, as a fan of existence.
I'm not convinced I'm right to think putting a dollar value on existence is folly -- but I can't conceptualize what it would mean.
And what if the cost of not doing that is also "astronomical"? Billions of people need to eat, work, raise their families. Somewhere between mass starvation/war and environmental destruction is a cost less than "astronomical" which people are willing to pay. It has to be realistic or it won't happen.
It seems like you might be missing part of this discussion: with widespread environmental collapse as described -- irreversible decline -- humanity faces extinction, regardless of the number of jobs in the economy.
Yes, but unrealistic measures won't motivate people to make the necessary changes. There are costs people will pay and ones they won't. You're not going to care much about the future if your kids go hungry now because there are no jobs. Particularly when it's based on a future prediction that might turn out differently.
if you can't put a real cost on something you can't actually weigh it, what happens if you say ecosystem damage is astronomical in cost and the loss of jobs if we don't build this dam costs 1.5 million dollars is the dam gets built because nobody has put any sort of measurable cost on what happens if the dam gets built.
Not disagreeing with you exactly, but given the global ecosystem predates money, is it really outside the realm of possibility that money can't be used to accurately describe or measure it?
"Astronomical" is the cost of putting ecosystems on an irreversible, downward spiral.