I get the horrible feeling that that tool's opinion boils down to nothing more than that he needs lots of plebs to dig in mines and put things together so he can make more money, and cause overcrowding that motivates more people to support his efforts to travel the void. It's very capitalist. Imagine we had a world with only maybe a few hundred million people, all that room to roam, all that abundance to enjoy. We might not want to work like dogs to make him billions or see the need to jet off to Europa.
Elon Musk, despite his more funny ancap moments, is like this weird cartoon version of a neoliberal under late capitalism, and by that I think he believes that capitalism works for the benefit of everyone and that a world without a perpetually growing labour supply and consumer demand would be one in which everyone was worse off.
The weird irony of this is that Elon is also massively pro-automation. He thinks that in the future people won't have to work because robots (presumably owned by him and his descendents) will do all the work, and everyone else will just sit around collecting their universal basic income and then immediately spending it on cryptocurrency or something.
I think we've always assumed that human civilization will grow perpetually and that future human or post-human civilizations will be enormous, but I don't think that's necessarily true at all. In the past, populations grew because most people had very little to do with their lives except have children, because people needed children to care for them when they got old, and because for the few people who did have meaningful choices having children ensured the continuation of their status and position when they died. It turns out, when you give people (particularly women) choices about what to do with their lives, having lots of children doesn't actually rate too highly.
I think population decline is kind of a natural and inevitable process at this point, and if our society wants to avoid it for some reason we need to look very closely at how to make having children more rewarding and less challenging.
I think there is a very utopian possible future in which, as you say, a few hundred million people live out their lives on an unspoiled earth, devoting all their time to leisure and intellectual pursuits while leaving the work to machines, and occasionally having a child whom the whole community will have the time and energy to care for and educate.