Doesn't work. You're essentially saying to assign the value of infinity. Then if there is a one in a trillion chance of ecosystem damage, you have to refuse even if the alternate is to definitely lose a billion lives. Can't be infinity.
At this point you're at the old joke. Man asks a woman if she would have sex with him for a hundred billion dollars. She thinks about it a while and then concedes that yes, she would. The man then asks the woman if she would have sex with him for five dollars. She's offended. She says, what kind of woman do you think I am? The man replies that they've already established that and now they're just negotiating on the price.
You can't just use an astronomical number. It has to be as accurate as possible because it's being traded off against other real things of significant importance. If the number is too low, bad things happen. But if the number is too high, different bad things happen.