Yeah a changing population has a variety of extraordinarily confusing effects

Certain resources would become more available such as land and housing, while others like say, new cheap speakers that rely on plentiful hands and brains, would become scarce. You may see people who are retired and rely on new speakers staying cheap use laws or force to maintain their same level of access to new cheap speakers, at the expense of everyone else. The newfound access to housing in desirable areas may also not be a panacea: with a decrease in hands and brains some neighborhoods may find themselves poorly maintained, abandoned, or even dropped from local utilities (like in Detroit when its population drastically changed).

In general I think, in the USA, a growing population is a good thing for our pie. There's some level of population where too many people is bad, but until then each person adds to the experience of everyone else.

Even for housing it would feel positive: if our population was growing quickly many wouldn't mind moving to St. Louis - today's St. Louis will be nothing compared to tomorrow's Mega Louis, for example!

No. "the pie" is total productivity, and "the pie eaters" is total consumption. Every human is both producer and consumer of this pie. The problem is not over/underpopulation, but the distribution of producers and consumers. Young people are more productive than old people, and that's why declining birth rates (and longer lifespans in retirement) are a problem. The scale is tipping to having more consumers than producers.