Are you also actively working with other people/companies who might hire those same people to put caps on what they might get paid, or indeed to negotiate hiring them at all? Google/apple/etc have enormous sway in some areas re: hiring/referrals/etc, and engaged in 'antipoaching' stuff that prevented free movement of labor (imo).

The plumber/mechanic analogies fall down when you look at the size and actions of google in the marketplace as a whole, not just with the specific interactions of an individual contractor/employee.