Humans having more more arch wizards than any other race has actually a very simple explanation beside population numbers and that is the most important human trait... ambition. Elves and gnomes are more suited to be wizards, but they generally lack the ambition that drives a human.
The floating ASI change affects more than humans, shield dwarves and half-elves... what's the point in having multiple tiefling subraces now? Just a different cantrip and 2 spells at 3rd and 5th level? Under the good old core system there was an actual difference between Asmodeus tieflings and Zariel tieflings, where one was more suited to casters and the other was slightly tilted towards melee. Mephistopheles tieflings were always in a strange situation, having the same ASI as Asmodeus tieflings, just different spells.
