> You imply that population will increase indefinitely

You imply that it won't.

I don't see a fundamental reason why humans would stop at 8 billion - yeah, currently, (my hypothesis is that) the world is kind of shit, so people stopped procreating due to stress. However, there's no reason to think that people won't procreate once the world becomes a better place to live.

> or that degrowth needs to happen forever

Assuming population keeps growing (which I see no fundamental reason to doubt will happen eventually), degrowth needs to happen forever in order to keep the consumption from growing - basic math:

    total consumption = number of people * avg. consumption per person

To keep total consumption constant, avg. consumption per person needs to decrease proportionally to the increase of the number of people.

> Also, slippery slope argument.

Not a real counter-argument.