First, GP wrote:

    > A problem solved if visas are not associated to employers, because then an employer couldn’t hold onto the employee like this.

Then you wrote:

    > No. Because it still floods the job market with off short talent that is willing to work for 30% less.

In many highly developed countries in the world, visas are not associated with an employer. We don't see people clamouring to post about it on HN. Why? Because the number of visas offered to skilled migrants is relatively limited.

Second, you wrote:

    > Construction workers arent tied to a single employer (usually) and that drops the price of labour across the board even in union dominated markets.

How can you be sure that this is true? Do you have a "natural" experiment where two similar areas in the US have construction workers where area A has workers tied to a single employers and area B not? Else, how can you say this with such confidence?