Why KDE keeps rewriting every single component every few years? Is that because of improper design/planification or because of bad implementation?
Where "few years" means 8 years? :D By all means rewrite software when it is 8 years old. A lot has changed in that time and the author notes that user expectations have changed, which leads me to the...
...false dichotomy you offered. User expectations and the code base not supporting those is the stated reason.
If KDE was a monolitic project with insufficient tests, those details wouldn't improve. Instead, I'd bet developers would be fast at removing current functionality when unavoidable changes add hard to track bugs.
And to be honest, I think the notifications area required a bit of optimization. To me, it feels like it is also one of the components that received more optimizations than any other component already, but it also seems to be a little more complex than you would expect on first sight.