I think it is generally well-understood that individuals more senior in their career have generally higher incomes. So, when it is stated that "the rich" are obtaining wealth gains, it is explicitly looking at structural changes rather than simply an earnings cycle for individuals continuing to be true. Your first sentence is "Adjusted for inflation, the US economy has more than doubled in real terms since 1975" -- but it was the case even in 1975 that there was absolute income mobility as one progressed in their career. So, your argument purports to disprove the relative mobility or inequality arguments of Saez/Piketty but in fact cannot.

That is why there is so much pushback on this piece of yours: because it appears to be attacking a strawman unrelated to real inequality in the system.