>

Your argument isn’t true, and you end up in a state of absurdity.
Population decline by itself isn't a bad thing. The rate of population decline is what is worrying. 
I live in a country that had just under 4.8 million people 30 years ago. According to last year census, we are just above 3,8 million now. By the end of this century we will be perhaps below 2 million. Whatever your political views, the idea that that sort of population decline wont have negative effects on the country and society is completely delusional.
And we are not an exception. Developed countries around the planet are going to have rapid population decline in coming decades.

Innovation comes from a well educated population. Not from a large population. 

If you live on a planet that has a bunch of old people, instead of young people, that means that a greater proportion of money and resources that would have otherwise been spent on education or innovations now has to go towards medical care for the old. Which means less innovation. 
Also. All other things being equal, would a planet with a million people or 10 billion people have more innovation ? The answer is obviously 10 billion. Larger well educated population will have more innovation than a smaller well educated population. Didn't think that was controversial. >


All other things being equal, would a planet with a million people or 10 billion people have more innovation ? The answer is obviously 10 billion. Larger well educated population will have more innovation than a smaller well educated population. Didn't think that was controversial.

The controversial part is where you need to gamble on those 10 billion people always being able to find technology that will save the day. Because if they don't, we are doomed.
On the other hand, if the planet only had 1 million, carbon emissions would be drastically reduced beyond our wildest dreams, and there would be plenty of resources for all. >



On the other hand, if the planet only had 1 million, carbon emissions would be drastically reduced beyond our wildest dreams, and there would be plenty of resources for all.


As someone worried about carbon emissions, do you know what percentage of the atmosphere is carbon?  And what percentage is the ideal? >

It's not that there's a particular number that's ideal, and the problem is that we've stayed from that number. It's that it's changing so rapidly that it's snowballing faster than life in general, and human society specifically, can adapt on a large scale. If the global average temperature rose by 5°C over the next millennium, it probably wouldn't be too big a deal. But half that over the next 100 years is a very big deal.
I guess if i had to give you a direct answer, it would be approximately the amount that we've spent the last few millennia learning to live in. Decisions from that are fine as long as they happen slowly enough that we can adapt socially, and other species can adapt genetically, to the change. >

The atmosphere is currently 0.04% carbon.
Are you saying that 0.05% too much and 0.03% the ideal? Or is 0.04% the ideal?  What happens if it doubles and reaches 0.08%? Catastrophe? >

You're really glossing over the whole "lots of old people, very few young people" problem. >

Yeah, OP says “people should be saving money for retirement, so lots of old people being retired shouldn’t be a problem”
This is a fundamental misunderstanding of how the world and the economy works. It doesn’t matter how much money people have saved for retirement if there aren’t enough working people to make the stuff they need to buy with that money.
It isn’t like saving for retirement means you are saving cans of food to eat later, like you are a squirrel burying nuts for the winter. You are saving money that you hope to trade in the future to someone else for your food and basic needs. This will only work if there is someone to trade your money to; if society is full of old people who have saved up money for retirement and not very many young people to make the food and things, then you are just going to get rapid inflation as all these old retired people try to trade that money to a dwindling number of productive people.
If this is hard to understand, just imagine the extreme… what if ALL the people are old, and they have all saved a ton of money…. Who are they going to pay that money to? No one is making food, they are all sitting on their pile of money wanting to buy food. The money would be worthless. >

ALL the people won’t be, so such an extreme is absurd. reductio ad absurdum >

I wasn't claiming the extreme example would ever happen, I was trying to show why having fewer working age people would lead to problems by using a hypothetical case of having no working age people... sometimes it helps to see an abstract thing like that by imagining the extremes.
Also, that isn't what reductio ad absurdum means; it is a form of argument where you show that if your argument wasn't true, you would end up in an absurd situation.