That's why I was very specific when I pointed out that the population growth has been slow. There was no confusion about rapid growth (although some of these states are growing quite solidly, Minnesota for example).
The parent was clear about the claim: Kansas and similar heartland states are losing population. It's false.
Over decades there's a massive difference between gaining 0.3% (or similar) population per year and losing 0.3% per year. Kansas gained 1/3 more people over 50 years. Had that gone the other direction persistently, they'd barely exist as a state today. Contrast that with Germany over that time, which has seen maybe 5% growth. Germany would have loved to have 33% growth over 50 years. Or the Baltic states, which are more comparable in size to a Kansas - zero net growth over 50 years. Now that's stagnation.
Or put it into more concrete economic terms. With the additional 1/3 more people, Kansas has an extra $38 billion in annual GDP output today, a total of ~$160 billion. Had they gently gone the other way, and declined slowly over time, it would be more like $60 or $70 billion today. It's a huge difference. They have a GDP per capita higher than Sweden, so when you can organically add 1/3 more people to that, it's a very meaningful economic result.