It will be years before TVs as well as desktop monitors are HiDPI in overwhelming numbers. 720p/1080p is pretty much the norm for those now. These fonts are meant to be usable on the vast majority (> 95%) of customer's screens, not just the minority (majority soon) of HiDPI screens.
> The only reason to not use a serif font is because at low resolutions, the serifs blur into a mess, […]
Nonsense. It's also an aesthetic call. We are not just using sans-serif fonts because our screens can't do better; sometimes they just work on their own merits (whether that's legibility or appearance).
This claim is often repeated but questionable at best.
>subjectively they're much prettier than anything else
They might be pretty to you (and this is such a subjective claim that using it as evidence for favoring serif fonts is pointless) but users are pretty used to sans UI fonts at this point, and there's 0 reason to alienate them with something "prettier".
1. Serifs often have uneven stroke weights which make them actually actually harder to read in small sizes (8-15), since the thin parts start to breakdown.
2. Letterforms are more complex and generally create more "visual noise" or the texture of the text gets noisy since you have lot of points and ticks on the screen.
I think serifs can work well on titles that are large, quotes, in print or simple ui like articles where you one column of text, mostly in one size. For UI, especially mobile UI feel like it can be quite jarring in any more complex app.
Legibility also comes from the different factors of the typeface (x-height, open counters, distinct letterforms) not only from the fact that it's serif or sans serif.
Actually, for reading long books some serif fonts will tire my sight faster than sans-serif fonts.
It seems to me font readability design is a lot more complex than simply serif vs sans-serif.
I disagree. With high-resolution displays becoming the norm, I'm questioning why so much of the industry is still stuck on sans-serif fonts for UI, and worse, regressing from humanist sanses to neo-grotesk fonts, which are the worst (seriously, if you have to use a sans, use something like Myriad or Noto Sans... which Apple and Google used to use before they jumped on the neo-grotesk bandwagon).
The only reason to not use a serif font is because at low resolutions, the serifs blur into a mess, but low resolutions aren't an issue anymore. And on top of that, hinting is a thing. Serif fonts have objectively better readability, and subjectively they're much prettier than anything else.
I've been using Noto Serif as my main UI font on pretty much everything for a couple of years now, and it's absolutely perfect. I'll never go back to a sans for UI.