I accuse those 15 inch laptops of being below the bar. 15 inch should be 1600x900.

If 960xwhatever is okay at 12 inches, then 1366x768 wouldn't even be the baseline resolution for 15 inch laptops, it would be the baseline resolution for 17 inch laptops. That just sounds silly to me.

Assuming the laptop screen is just 20% closer goes a long way here to figuring out a good resolution. And it gives 720p to 12/13 inch laptops at 1x.

I guess, but even without measuring pixel inches/degrees it feels clearly wrong to me to say that proper 1x on a 12 inch laptop screen is only 960x600. 1280x720 or 1280x800 makes more sense to me, and then there's no confusion because 1920 is a clear 1.5x resolution.

PPI doesn't generalize across different types of display but it works pretty well within a category of monitor, laptop, tablet, phone. For TV you probably just assume it's 4K and figure out the size you like.

It's wrong but it's wrong in a way that causes minimal trouble and there's no better option. And if you add viewing distance explicitly, PPI+distance isn't meaningfully worse than PPD+distance, and people will understand PPI+distance better.

You could say it masks over the uncertainty in some ways, but it doesn't introduce that uncertainty. Asking for a laptop with 100PPD doesn't even make sense.

> the 20/20 visual acuity threshold

The acuity threshold for random blobs of light.

The threshold for sharp edges is much finer, and the things we put on computer displays have a lot of sharp edges. https://en.wikipedia.org/wiki/Hyperacuity

But nobody knows what baseline PPD is (47) and you can't actually specify a laptop screen in PPD, you can only specify it in PPI. So I think it's reasonable and maybe even preferable to use PPI here.