H1-B visas are dual purpose; they support intent to transition to permanent residency, and have a defined procedure for the same. In particular, I don't think

H1-B visas are dual purpose; they support intent to transition to permanent residency, and have a defined procedure for the same. In particular, I don't think many skilled employees with a global choice of employers are going to move to a country for a 3 or 6 year stint with deportation waiting at the end of the period. The costs and trouble of moving don't make it worthwhile, and it's hard to predict what life will bring - it's best to maximize all options. So I think anyone migrating to the US on a H1-B visa will also end up going through the permanent residency application process, just so they don't face a cliff edge after 6 years. That would make them migrants at the start of the process, rather than some kind of seasonal workers.