Yes, there are people at the extreme ends whose spending vs. saving behavior can't be influenced (much) by the relative favorability of investing. But there are also lots of people who will respond to this incentive, and we should be concerned about what effects that might have.

Your post belies an assumption that the new tax revenue would actually be spent on the people for whom "it's not a fucking choice." But that's orthogonal. Congress is perfectly happy to spend new tax revenue on tax cuts for the very rich, on the military-industrial complex, on seniors of all income brackets, on predominately upper-middle-class students, etc. "What are the right ways to raise revenue" is a different question from "what are the right things to spend it on," and having a compelling reason to spend has no bearing on the virtue of any particular method of raising the funds.