Well, the earth isn’t a closed finite system for starters (there’s a continuous input of 173,000 terawatts of power), so there’s that. Resource and land utilization have become dramatically more efficient over the last several hundred years, and there’s no shortage of inhabitable land, so there’s no immediate limiting factors, particularly if technological development continues.

Carbon production is an obvious elephant in the room, but even if you stop population growth today, there’s still a very large problem that must be solved by radically changing our energy systems. Limiting population growth helps a bit, but it’s not anything close to a sufficient solution.

The fact of the matter is that it’s not so much that there’s some obvious limit of resources that we’re hitting in so much as the amusing truth that, if you give people a generally decent life and the ability to regulate when they have children, most people don’t actually want that many. That’s the primary thing behind declining population growth, and you observe it in essentially every country that becomes economically advanced (relatively speaking).

While I’m at it, capitalism as a general model doesn’t require infinite population growth either, only some continuous economic growth, which can come from technological advancements, new product categories, changing interests, or yes, more consumers. Given that a non-trivial chunk of the global population are still trapped in subsistence farming, there’s no shortage of economic opportunity.

Edit: Things like welfare for the elderly are much more damaged by population declines though, it should be said.