You say that, but the US per capita GDP, adjusted for inflation and PPP and anything else you care to throw at it, has only increased since then, and not by a small amount.
When I look at median income over time versus per capita GDP over time, adjusting both for inflation, they're both increasing, but they're also diverging because the bulk of the gains are concentrated at the top.
That is as big a reason people are having a hard time making ends meat nowadays as anything. Unfortunately, fixing that trend will require reversing most of the tax code changes made in the last half century and a lot of aggressive anti trust action - i.e., breaking up markets allowed to consolidate too much.
I don't see any of that happening without a clean sweep of DC, though.
Oh I completely agree that GDP is a terrible measure of prosperity (and even median income is lacking, as you noticed). But I was not using it to measure individual/median prosperity, but the overall material wealth of the country, to debunk the assertion that a single teacher's salary could comfortably sustain a family then only because of a unique, brief set of circumstances that materially enriched the US.