But what if the root cause of a middle class decline is "routine" jobs being automated and outsourced? That's what my lay-reading of the Economist over many years tells me, though I'm not in the field.

In that case, tax policy may dampen or slow the decline, but won't fix it.

I agree with almost everything you say, but I think your first two paragraphs only imply that expansion is relatively insensitive to tax rates (so republicans' tax paranoia is unwarranted).

But is it good evidence that raising tax rates today would substantially improve the situation of the middle class? I don't think it is.