Some of these concerns aren't valid (for instance: the baseband of your phone is less powerful, in its systems design, than is supposed by open hardware advocates), but mostly the issue is: security is about costs and prioritization, not absolutes. The systems people propose instead of Signal are likely to cough up secrets directly to adversaries; switching from Signal to something else reduces security.

If you believe people with state-level adversaries should exclusively use transparent and open hardware, that's fine; you should see Moxie's comment on this thread to learn how you might go about ensuring that the tiny minority of users with that hardware have access to Signal on it.