I really disliked previously, when icon prominence could be wildly different because one icon takes up the full area with a big square, while another is a circle that necessarily has a significantly smaller area within the same extent. Icons from Apple were all nicely balanced in size, but third-party apps could be anything.
Giving equal visual weight to each icon is an improvement. iOS was a step forward in this direction, and now they finally brought the same standard to Mac.
Squircles aren't ugly, they're functional. "Shape" hasn't disappeared as a distinct visual cue, as the area within the squircle is made of, well... different shapes.
And let's not forget the fact that Macs still effectively use icon masks. A smaller icon is harder to click, because clicking on a transparent area... doesn't click at all. I remember icons like a skinny letter "S" that you had to click just right or you couldn't at all.