The problem with the shift to flat design wasn't that it stopped being "fun", the problem was that there was a whole bunch of things being communicated to users by the previous, non-flat designs that got lost in the transition, and that flat design doesn't have a good alternative for:
- Not using shadows and lines to separate panels, and using solid color blocking or whitespace instead is much more confining, design-wise, and greatly limits possible information density. Sure, all that whitespace looks great in presentations, but it's like the difference between a designer kitchen from a magazine and a place where I can actually cook.
- Panels that are raised up or recessed using shading provide an obvious hierarchy, giving users valuable cues as to the relation of the elements. With shading and depth, it's obvious that items in this toolbar affect the items in this panel below it, one is subservient to the other. With color blocking, it's all just rectangles next to each other.
- Buttons that are slightly raised up have an affordance for clicking, and there's no way in hell you can confuse them for a label - it's much easier to overlook that 'the label with blue text' is actually a button, while the label with black text is just a label.
- Ostensibly, a flat design contains fewer distractions. But ironically, in reality, a human brain may have a much harder time understanding an interface when everything is flat and looks alike, than when elements have depth and distinctness to them. More information communicated does not always translate into more cognitive load in processing it - often the opposite is true.
Sure, I understand the urge to rebel against skeomorphism (that faux-leather in the iOS Find my Friends app was an abomination) but they threw the baby out with the bathwater. I'm glad to see a lot of these changes being slowly walked back with successive versions of iOS - in the linked article the comparison between the "tabs" (segmented control) in iOS 12 and 13 really showcases how a shaded design can look both less distracting and convey more information than a dogmatically flat design.