Obviously it can be difficult for poorer people to flee an oppressive state, it does take resources to move interstate
Although this is true, it is the threat of people moving that will be the primary deterrent. States will know that their most productive citizens can leave and they will foster policies to protect against that. Poor people will thus benefit even if they can’t easily leave.
It’s similar to local grocery store prices. Most customers don’t go to multiple stores comparing prices, but the fact that some do and anyone can generally forces the local stores to maintain competitive prices.
Oh, that's a really good point. I keep forgetting to factor in the threat or the potential of eventualities to incentivize good outcomes.