More pixels only addresses the blurriness caused by a display's low resolution. The other side of the problem is the limitations and quirks of human vision, which require a different set of adjustments to increase legibility at small sizes. And arguably, increasing resolution can make everything smaller (esp. if there isn't a 'High DPI' setting, and so makes the size problem even more urgent.
The design adjustments needed by icons at small sizes are closely related to optical sizing in fonts (increasing x-height, lower contrast, heavier weight, large terminals and serifs are shrunk, small ones enlarged, etc.), except that an icon's topology ofyen needs to change as well.
A simple example is that if a large icon features a 3x4 grid as a design element, at small sizes it may need to be a 2x3 grid instead, the point being that the grid-ness of the design element is the important part, not the specific number of rows and columns.
Other design elements may just need to be removed entirely (such as digits or division markers on a clock face).
These sorts of 'optical icon adjustments' (I'm not aware of any other term for them) are really important for icons, perhaps more important than optical adjustments to fonts, since icons are less familiar and so are harder to recognize.
In summary, increasing the resolution of the display only solves half the problem (and often makes the other half harder).
If you have a Mac, try making the Dock the smallest possible size. Those icons are just scaled down from the larger versions, and they can be hard to make out sometimes.
Even on high-res displays, small versions of icons would benefit from separately made versions that leaves out some detail.
