The tax cuts and other regulatory shifts allowing things like share buyback and the carried interest loophole (might be closed now) have shifted up the share of the GDP held by the top 10%. This has slowed the velocity of income, as a wealthy person doesn't spend their income the same month the way a person in lower quin-tiles does. This has caused a stagnation in middle class standards of living. This is a headwind that didn't exist 60 or 70 years ago. There was also a demographic advantage as WW2 had thinned the heard and there was a once ever level of economic prosperity. The tax code encouraged big business to invest in labor, which isn't happening now.