Recreating Aqua is the easy part. Recreating all the applications you would use day-to-day to fit the design language specified by Aqua is another. Apple's visual OS design was never that far ahead of the curve, but they managed to convince developers for their platform to stick to their guidelines rather than reinvent the wheel, making the entire computer feel more like one integrated system than a toolbox filled with differently branded tools.
This is also why most "windows style" themes fall flat: you can copy the window decorations, button backgrounds, and icons, but unless your applications are designed to look and work like the OS your mimicking, it'll all just look weird and off.
At this point "operating systems" in a commercial sense are so large that only relatively new entries can afford to rebuild their stock applications to fit the current UI theme (ChromeOS comes pretty close but you'd need to appreciate Google's design to enjoy that). macOS, Windows, and even Linux to some extent all have decades of old software to support so they can't redesign their core GUI stack without breaking everything.
In the days that an internet browser wasn't considered a core part of the operating system, there just weren't as many places to get the design wrong or off-template without Q&A noticing.
This is also why most "windows style" themes fall flat: you can copy the window decorations, button backgrounds, and icons, but unless your applications are designed to look and work like the OS your mimicking, it'll all just look weird and off.
At this point "operating systems" in a commercial sense are so large that only relatively new entries can afford to rebuild their stock applications to fit the current UI theme (ChromeOS comes pretty close but you'd need to appreciate Google's design to enjoy that). macOS, Windows, and even Linux to some extent all have decades of old software to support so they can't redesign their core GUI stack without breaking everything.
In the days that an internet browser wasn't considered a core part of the operating system, there just weren't as many places to get the design wrong or off-template without Q&A noticing.