If designers need to justify their jobs, fine. They should design the interface layout with modular components that can be entirely customized by the user.
IMO no user should ever be forced to use designs that are the product of meaningless fads in the design world.
Maybe developers are justifying their jobs. Maybe younger developers are excited about ditching the old crufty frameworks and languages, and exploring something new. Maybe every 10 years they reinvent the old wheels, and older developers are grumpy that the change was not needed in the first place.
The backward compatibility effects aren't quite as bad in the browser context... except when the plugin API is affected.
You can see it in this article: https://9to5mac.com/2021/06/16/safari-in-macos-monterey-what... (image: https://9to5mac.com/wp-content/uploads/sites/6/2021/06/how-s...)
Hopefully, they add back some of the missing customization options in a later release.