While I think you're right about the immigration, what I remember from Enrique is that no/hard any non-Betans would qualify for the Betan Astronomical Survey if it weren't for affirmative action programs. This in the context of his being really impressed by Cordelia. Beta doesn't need to import any smart people, it produces plenty of its own, but they apparently cultivate foreign diversity anyway -- in the thing that's their closest to a military.
Uterine replicators are only 200 years old in the universe, and it's about 3000 AD, so we've got 600-800 years of colonization to explain without it. OTOH France and the Nordic countries are having upticks in birthrates, and among the highest in Europe, so enough social democracy may in fact be what you need. But yeah, to get colonial-high birth rates we need somewhat different culture or circumstances than simple extrapolation makes likely.
Bujold's also said that she does worldbuilding on an as-needed-for-story basis, and that she reserves the right to Have A Better Idea. I haven't seen her say what you attribute to her, that sounds more like Babylon-5.
Re: Beta Quibble:
Uterine replicators are only 200 years old in the universe, and it's about 3000 AD, so we've got 600-800 years of colonization to explain without it. OTOH France and the Nordic countries are having upticks in birthrates, and among the highest in Europe, so enough social democracy may in fact be what you need. But yeah, to get colonial-high birth rates we need somewhat different culture or circumstances than simple extrapolation makes likely.
Bujold's also said that she does worldbuilding on an as-needed-for-story basis, and that she reserves the right to Have A Better Idea. I haven't seen her say what you attribute to her, that sounds more like Babylon-5.