I'd mostly agree with you on this point and it's divergent from the one that the Trumpistas care about.
That said, I have seen companies that were "H-1B dependent", ie, they have paid the fees for visas to hire from outside the country, because they were able to get lower salaries from doing so even after paying the legal fees. Clearly not the Googles of the world, but they do exist.
Yeah they definitely exist. And the ratio of those vs. the Googles will affect how elastic it is. Anywhere between 0.1 and 0.9 for the elasticity sounds plausible to me (not an economist, I could be not even using the right concepts, so take that for what it's worth).
That said, I have seen companies that were "H-1B dependent", ie, they have paid the fees for visas to hire from outside the country, because they were able to get lower salaries from doing so even after paying the legal fees. Clearly not the Googles of the world, but they do exist.