Earlier quoted context omitted.
IMO the default for body text should be serif, with sans used for headlines and display text. There’s a reason books and magazines are set in serif fonts: they are easier to read for long text paragraphs. GUIs defaulted to sans serif fonts because displays didn’t have enough resolution to properly render serifs, but that’s mostly not the case anymore.
I don't think there's anything inherently more legible about either serif or sans-serif fonts. Whichever one you read the most will be most legible to you, which means the default shouldn't be changed even when it's no longer technically necessary.
As an example: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4612630/