It feels to me that a lot of the bigger ideas in KDE fell away over the years. In the 2000s I would log in every morning, open a KWord doc in one Konqueror tab, a KSpread sheet in another, and some browser tabs alongside them, then I'd launch Kate and open some files over SSH or FTP and get to work. It felt like someone had really embraced OO and applied it to every part of the desktop, and I assume something like KParts and KIOSlaves still exist. But for the most part, I use KDE now as a bog standard boring Linux desktop that just works. I am grateful that it hasn't been dumbed down quite as much as GNOME over the years, but I hope they have a few bold experiments left in them (and would love to hear what I'm missing if it's already there!)
All the development action went to the web. Dolphin's still pretty awesome.
I still find a decent amount of the integration, like KIO, is still there and works well - it puts MacOS and Windows to shame in terms of how I can just interact with files anywhere as if they're native within KDE apps.
It's kind of a shame that Konqueror fell to the wayside, but modern browsers are so complicated I cannot fault them for focusing elsewhere.