> The policies of supply-side economics, however, are much more defensible, but it seems people would much rather pick on the strawman.

Lowering taxes is one of the policies of supply-side economics:

> Supply-side economics is a macroeconomic theory postulating that economic growth can be most effectively fostered by lowering taxes, decreasing regulation, and allowing free trade.[1][2]

* https://en.wikipedia.org/wiki/Supply-side_economics

Along with decreasing regulation and allowing free trade, so you've just knocked down one of the reasons to think about supply-side stuff seriously.