(A) The fact that silicon valley gets access to global talent which make sthe companies much more successful and massively increases the number of jobs available providing far more jobs for US citizens than if the visa program didn't exist
(B) The H1B visa holders are earning far more than they would get by remaining in their home country, and whilst they're bonded labour in the US they can always return to their home country leaving them at a minimum with a choice between a better job in the US or the same situation they had in their home country anyway.
(C) The US gains a bunch of highly intelligent, highly taxable employees who will likely eventually become citizens and can then create startups as well as providing a larger labour pool for the people who are starting companies.
All of your critiques are comparing H1B visas against some imaginary system where people can come to work in the US with no restriction, and let's remind ourselves these changes are from a thoroughly anti-immigration administratrion. The alternative to the H1B system is no H1B system, which is why it's been so difficult to reform. There is no law getting out of congress that reduces the restrictions on immigration.