Trying to understand where this is coming from, I guess two sources:

1. It's a fashion update to give GenZ and younger something they haven't seen before. They are too young to remember Windows Vista, and are the most important future target group that spends 12+ hrs / day on their iPhone. Also it is an audience that heavily customizes their UI, and care more for visually communicating cool-ness, than to get work done with efficient UX. Similar to using rainmeter on a desktop PC. Unsurprising, this look a lot like a rainmeter skin.

2. This is a way to communicate unmatched quality. Similar to what AirBnB are doing. When everyone can use icon- and component libraries like material and shadcn to build UI:s, this is a visual language that communicates premium quality is through an interface and iconography that is different and too expensive for others to recreate. Many companies don't have the skill nor the time and money to do custom icons in 3D software, or create elaborate translucent effects. Let's see what multi-plattform apps will look like with this new UI, perhaps the goal is to make them stand out as "outdated"