Full disclosure: I kept KDE 3.5 running, held together with duct tape and twine, until about 2014. For the past few years I've been mostly on Windows due to work requirements. Just yesterday I installed Debian on a box, and booted into the latest version of Plasma.

It's slick and fast, I'll give it that. Reasonably configurable, although still not to the extent of 3.x. What I find most disappointing is that the integration still isn't remotely on a par with 3.x, and it doesn't even appear that anyone is trying. Remember when all the apps talked to each other, and were accessible through Konqueror via Kparts? Remember being able to seamlessly manage multiple workflows via a single interface? Being able to play a music file straight from Konq without opening a separate app? Is this a concept that has been completely abandoned? I can't help but notice that mighty old Konqueror is now so undermaintained it's damn near deprecated.

Is this a deliberately chosen direction, or something that just kind of happened? Am I the only one who even cares?

There has certainly been some change in direction in this regard, with a general preference towards dedicated, purpose-built apps such as dolphin, okular, falkon, kate, etc. However, Konqueror and the kparts system are still around and you should still be able to open different apps inside of Konqueror (not sure if there are some special configs you need to enable to get it all working again, but you probably need to start by going into the Default Applications settings in System Settings and switch the default file browser from Dolphin to Konqueror).