It’s not a loop, it’s a spiral. Each idea brings a (partial over-)reaction to the last one.
For example, flat monochrome icons emerged, because previously the UI was massively overshadowing the main content [^1]. So we traded the recognizability of icons for a better overall hierarchy in the UI.
Now that this problem is solved, designers are looking to reintroduce recognizable icons without sacrificing the previous goals. In the AirBnB app, you’ll find that the busy icons are only used when they’re the main focus. Auxiliary icons remain flat.
[^1]: https://www.geeky-gadgets.com/wp-content/uploads/2011/09/fac...
Flat, monochrome icons emerged due to the hardware requirements of mobile devices with wildly differing DPI.
There can be multiple reasons. But iOS went 6 years with skeumorphic icons (post-iPad, post-Retina) so mixed-dpi screens probably weren’t the only factor.
At that time iOS had the luxury that its higher-DPI variant was exactly twice the original resolution, so up- or downscaling of non-vector graphics worked well. (Similar for macOS.) Moreover, the original iPhone resolution was already relatively hi-DPI, meaning that accuracy on the individual pixel level wasn’t as important as on the typical 96-DPI desktop screen.
