tldr; turnover of visa holders vs. non-visa holders should be used as the metric more than wages. Significantly lower turnover of visa holders suggests they're being exploited since those who can quit and find another job do.

But this doesn't work. For many people on H1B's the visa and greencard sponsoring has tremendous value. The do depress wages, but it's because of the risk around the visas.

If you have an H1B, you're less likely to work at startups -- if the company goes bust and you need to find a job fast, you're at tremendous risk of needing to leave the US. You have 4-5 weeks to get a new job, but it has to start by then. Thinking about how long interview cycles can be at some companies, 4 weeks from application to start date is challenging.

Also, people getting sponsored are much less likely to leave. This should be the real metric for evaluation -- turnover. If you're on a visa, especially if you're getting sponsored for a green card, you're much less likely to quit and go to another company. You can get hired at a competitive salary, but over a few years and a few promotions you can be underpaid.

To completely stereotype, I used to work with a ton of these guys at Amazon -- they're all in line for Green Cards and will not consider quitting or even complaining about things until they get it. The turnover among non-visa people is much higher.