I remember when my Mac Plus's 72-pixels-per-inch display seemed like it would be all we'd ever need.

The size of a pixel had remained pretty much constant for 10 years. We had spent so long knowing that our pixel was roughly the smallest dot we could see at arms length, that we took it for granted. We used it as a unit. We used it all the time. We used it in our raster graphics for logos, we used it for buttons and icons in our software. We used it to get perfectly aligned layouts in websites. We used it to decide the size of our text. We knew we shouldn't but we did it anyway.