I am not sure about the "crucial" part, regarding having growing billions upon billions of people. Obviously you need some people to have economy, but what they do is more driven by incentives than essentialist features like being older. What I'd argue is that there will be a need for different business models, with profitability arising mainly from things different than offshoring and relying on dampening wages. If there will be money in that (not as easy as today's business models, but still), it will happen.

To make this happen we will also need, obviously, technology advances and organizational innovation. But to get more people for doing that, I would rely more on taking knowledge people out from industries becoming less profitable (a normal economic thing; many people would agree that currently smart people are attracted economically to many questionable enterprises) and lifting the developing world (as its population growth is slowing) than, as I said, growing raw population count.

I'm assuming you don't mean that we absolutely need a pool of people willing to work for any wage due to fear of extreme poverty. I have a sneaking suspicion that this point might hide under the "we need more people for more ideas" argument in the article. But the reality shows that we don't need such a poverty pool for agriculture for a long time now (looking at the Western agricultural productivity) and we probably don't really need it for industrial production either. This is due to technological advances letting us produce more with less people.

Do historical population declines bear this out? The Black Death is often credited for an increase in worker wages and a shift to a somewhat freer society with more freedom of movement and urban development. I can just as easily see that global population decline would lead to higher wages, immigration reform, more urbanization, development of automation, less focus on planned obsolescence and more on higher value goods that a wealthier population can now afford.

That didn't work out for India and Africa. They would be doing better with less people.