> This does not seem to be the case, at least not when observed over long enough time periods to filter out the noise. Looking at the earliest income records we seem to have, incomes were on par with what we find today.

I can find literally no data series that supports this as a general long term trend though there are some medium-term windows (from the late 70s peak to sometime in the runup to the dotcom boom, for instance) where it would have been true, but I’d be happy to take a look at your data.