A few months ago John Quiggin posted this and I find his argument pretty compelling

Are pronatalists living on the same planet?

But even in this extreme case, world population in 2100 only falls to 6 billion, the same as in 2000. I was around at the time, and did not feel as if there were too few people about.

What about the need for workers? One unsatisfactory feature of long-running projections like this is the use of outdated statistical concepts such as the “dependency ratio”, that is, the ratio of people aged 15-64 to everyone else. That made sense 50 years ago, when this range represented the period between leaving school and retiring in most industrial societies. But these days (and it will be even more so in 2100) education continues well past 20 and retirement is often deferred to 70 or more. A look at the age group 25-69 shows that it is going to remain more or less stable in absolute numbers declining only marginally relative to the growing population

Also followed up with this

A billion people would be plenty to sustain civilisation …