1) This is a particular case where making immigration difficult to the people involved directly results in less jobs for Americans, as observed above. Yet that's still what is being done. It is clear from it that, for some people, the desire to reduce immigration is not just related to job security. This doesn't mean there aren't legitimate concerns, just that it doesn't seem to me that the explicit legitimate concerns are the only issue here.
2) The particular cohort of H1-B developers I am more familiar with compete with American grads for equal jobs and equal salary/RSUs, and are nobody's 'cheaper replacement'. Now, I am not saying that what you describe doesn't happen. But you know what would help there? Making it easier for people in H1-B's to change employers or even stay unemployed (even on their own money, without benefits) for a few months in the U.S. searching for a job if their employer is exploitative!
There is a sort of issue 2.1. here that I will acknowledge: yes, having more skilled people come in increases supply and lowers prices for labor, slightly. But most of the jobs in question are at companies that target a global market, this means not making use of a global labor pool is both ideologically suspect and a bad long term plan. Talented people in other places will still compete with you, and now you don't even get their taxes or the effect their talent creates in the available job opportunities in your area. If, say, you keep all the Amazon jobs for Americans but end up buying all your stuff from Alibaba the world over, was that a good outcome for you?