“I’ve read from credible sources that even without advances in technology we could support 30 billion people on Earth.”
Do you have a link? It seems implausible to me. Two obvious questions are: At what standard of living would they be supported (as anthropocene_x pointed out)? For how long could 30 billion people live comfortably on a finite planet before exhausting nonrenewable resources?
Also, if current growth rates continue, the human population will exceed 30 billion before the end of this century. Then what?
If growth must be curbed sooner or later, better to do it sooner, and better to do it through some means other than a mass die-off.
At what standard of living would they be supported
Probably not a great one esp. at the one trillion level.