In a certain sense, Platinum was an attempt to reinterpret what Mac OS could have looked like if it had always been designed for a color display. It didn't just add color, like System 7.0 had; it added depth and texture to the interface which wasn't practical to display before. It also added a ton of new controls to the toolkit which previously didn't have standardized implementations or appearances. (For instance, System 7.0 didn't have a standard progress bar control - every application which used one had to provide their own implementation.)
A downward trend since 1991?
It’s fair to say that design has moved on in the last 34 years. Totally subjective whether you think it’s all been for the better. But macOS is self-evidently more usable now than it was then; a lot more people are using it. I imagine fairly few of them would be happy if Apple decided to abandon this Liquid Glass idea and return to System 7 design instead.
Apple's current design language is sterile, but at least it's easy to read. The modern design trends are just a series of downgrades in usability, arguably continuing since System 7. Somehow, it looks like "overlapping low-contrast window content" has become the haute couture of UX, much to the dismay of grandmas everywhere.