With ANY design language, people will look back and cringe. That's just the nature of how design trends evolve over time. And things tend to be cyclical as well.
Look at how architecture is viewed over time. In San Francisco, Victorian architecture during the 1950s and 60s was viewed as gaudy and ugly. Many home owners wound up replacing the Victorian detail with asbestos siding! And during the 80s, Modernism was viewed as overly stark and ugly. Especially mid-century Modernist buildings got leveled everywhere. But, now things have gone full cycle again and Modernism is back in vogue.
With graphic design, remember when Photoshop first came out with all of the texturing filters? Soon afterwards, print layouts were festooned with background patterns and shadow effects. It got to a point where many printed publications became unreadable because the titles had so many 3D effects applied, and typefaces went into overload. it was the logical outcome of a trend going to its logical extreme.
OS X, with its candy-like colors and textures, pinstripes, and metallic brush effects were an outgrowth of that design trend. Steve Jobs even described OS X as "lickable" and meant that as a positive.
By the time iOS came out, designers were already pushing back against excessive texturing and 3D effects, and the return to flat graphics had already begun. Subsequent versions of OS X saw Apple scaling back to a more minimalist scheme. iOS and OS X are following the broader design trend back to more basic layouts and references, i.e., the flatter look that prevailed in graphic design before texturing became all the rage after Photoshop.
In hindsight, any trend will eventually become dated. And when people yearn for something "new" and "different," it often entails bringing back the past.