Cricket, on techreflect.net:
Although they look visually similar, the Stage Manager interface is definitely much more refined and compact. Part of that is because shrinkydink pre-dated retina displays, so there was only so much shrinking you could do before it just looked awful (or more awful). Without the green-light, it never got sufficient attention from the Human Interface team.
Post was deleted, here’s the internet archive version. It does make appreciate the feature more knowing it’s been around as a concept for a few years.