I don't think a million will actually happen--the fight over declining resources will probably do us in. It doesn't really matter, though--they are both catastrophic. You accept that we eventually go down and are after kicking the can as far as possible--but that approach ensures that we will crash. We are looking to avoid the crash entirely.And it appears that you think the policy that will maximize the total number of people-years is to have a population above current levels until supplies run out, then it all comes crashing down and there are less than one million people left.1) I place the value in people-years, not in years. If a crash is inevitable, 10 billion for 150 years is superior to 1 billion for 1000 years.
2) The sustainable level is early stone age with a sub-million population. We have already taken all the easy-to-get resources, a low-tech society would already be on their way to a crash. The existing stuff will last for a while but it will wear away eventually.
We are trying to avoid the crash, not minimize it.And you appear to say that could happen in decades or a few centuries.
It seems to me that this policy causes immense unnecessary human suffering. If we had time to react by each averaging one less child then we are currently planning on, then we could begin reducing that population before we get there. If we reduce to two billion people over the next 150 years, I would think we could still live at a better standard than we would live at with 10 billion people. But even if the standard of living went down, it would greatly extend the time until the crash occurs, and make the crash much more tolerable when it happens. We might have time to even do a controlled descent to 1 million if we needed do I think we owe this to our descendants.
If we are recklessly driving into the wall at full speed, knowing that it is our descendants, not us, who will be in the car when it crashes, that does not seem like a humanitarian thing to do.