Here's my two cents annecdote. I'm currently running Plasma 5.12 on openSUSE Tumbleweed. I have a computer for my wife that runs Ubuntu Mate. KDE feels right in line with the speed of Mate. That was not the case in the 4.xx series or early 5.xx series, but since around Plasma 5.8 it has been exceedingly smooth. Now are XFCE/Mate type desktops perhaps a little lighter? Maybe. But I'd think you would be pleasantly surprised by the speed of KDE these days. But if XFCE is your style then no reason to switch.

I used to use Gnome and some of what you say is true, especially the resource demand of Gnome. More recently I've been using KDE on openSUSE Tumbleweed and it's been awesome. They have really smoothed over many of the roughest edges and tuned it up to be very light on resources. It offers compelling features such as KDE Connect to link up with your mobile Android phone and is very slick in that regard. And KDE is definitely nowhere near my-way-or-the-highway territory. It is extremely customizable and has become more intuitive in how to do so in the 5.xx series. Finally, I'd say that the desktop in general is a shifting paradigm, but still an important component. I think they are as relevant as any other desktop setup in their respective ecosystems. With containerization and things like Snaps and Flatpaks becoming a simpler way to distribute apps on Linux, the desktop has many compelling things to offer a user.

I've used pretty much every major DE for extended periods of time. I've been on Gnome 3.x for a decent while now. However, when I first got into Linux KDE was what I was using. I really liked the features and ability to customize it. KDE 4 had promise, but was a bit of a mess because the distros rolled it out too soon and it had some issues that were problematic for large groups of users (graphics on Intel and Nvidia hardware for example). Plasma 5 has fixed some of these issues, but I still find the problems I've had in the past with KDE are things that are still there. For all the power KDE can give a user, I've never thought that KDE did a good job of streamlining the experience so that average users weren't assaulted by all of the advanced features. I consider myself a fairly advanced user in terms of experience and understand of DE concepts and every time I've used KDE I end up suffering from feature fatigue. There a so many dials to turn in KDE and each dial has multiple switches that it seems endless. And things that should start out being simple like the file manager are, to me, way over the top. Most other DE's are setup with the basic folders along the left column (documents, pictures, music etc.), but in most KDE distros I've tried I have an assortment of smart folders and smart searches. Those are great features and can be very useful to power users, but aren't something an average user would likely work with. The whole concept seems very foreign compared to Gnome, Unity, Mate etc defaults. That's not say KDE should just copy what everyone else is doing, but some things I think are needlessly complex. I really do wish KDE the best. They have great tech, passion about OSS and an excellent community. And I don't want my criticisms to be taken as bashing. I am well aware that Gnome isn't perfect and has issues of its own. I just see the potential that KDE has and want them to find greater success. I guess my bottom line is that I find it easier to dial up other DE's instead of dialing KDE down.