I'll disagree with this. The balance should explicitly not change as you advance in levels. In other words, race and background should advance along with level and keep pace.I think the question is flawed. The percentages should change with level.
When I start playing at 1st level, I think Class and Race should each be around 40-45%, with the remainder being Background. By the time I hit 20th (or 30th, or whatever the highest is), I think Background should have receded to less than 5%, and Race to maybe 20%.
Advancement should matter.
I'll disagree with this. The balance should explicitly not change as you advance in levels. In other words, race and background should advance along with level and keep pace.

(Dungeons & Dragons)
Rulebook featuring "high magic" options, including a host of new spells.