The neutral: day-to-day things are unlikely to be affected much. Fewer farmers, but fewer mouths to feed. This applies to most consumables.
The mildly negative: for knowledge intangibles, a lower total population means a lower number of scientists, entrepreneurs, etc. Perhaps substantially less. Apex projects that take a substantial proportion of the world's capital and labor (e.g. colonies on Mars) become a lot harder, with many becoming impossible. This is somewhat counteracted by most countries becoming richer and more educated.
The disastrous: older societies where each person working has to support multiple other people are sclerotic societies. A society where younger workers have to pay taxes 2x higher than they do today, to support a bunch of old people who already own all the housing and capital stocks? That's a society that I'd move away from in a hurry, regardless of if I'm old or young. I'm going to have to encourage any kids I have to study Yoruba.
This is one of those huge issues on par with climate change, but people mostly don't care or think about it. I'm not sure what governments can do about it. At some point I want to study Israeli demographics and fertility more, as it's the only country I know of that has maintained a high TFR (>2.1) even in its secular populations.