I don't think it's that simple. In a setting where there is little concentration of wealth, and where it is also hard or impossible to borrow money for product

I don't think it's that simple. In a setting where there is little concentration of wealth, and where it is also hard or impossible to borrow money for productive purposes, I can easily imagine giving money to the "supply side" leading to large gains that "lift all boats" compared to not giving that money. With the benefit of hindsight, I don't think we've been in a situation that resembles that (at least in the US) at any point since "supply side economics" was formulated. I don't know what I would have thought at the time; I expect it was less obvious than it feels now. There's remains the possibility that, even in a situation like that, some other approach would work even better, although naively pumping money into the "demand side" probably isn't that approach.