I just spun up a linux Desktop machine for the first time in almost a decade and agree that Plasma has come a long way but it's still rough around the edges and, well, slow. Not as slow as the Windows UI, but still slower than xfce or the like.
One problem I do have with Plasma (though this may just be how arch packages it) - Dolphin, the native plasma file manager, is not included by default. Yes, I know it's not much of an imposition to do pacman -Sy dolphin but a file manager seems like an integral part of the system to me worthy of being included in the base package for a desktop environment.
Little choices like that affect new user adoption.
Try it on openSUSE, the best KDE integration by far IMHO, since it is their standard DE since IDK/forever? Tumbleweed offers the newest packages, rolling like Arch, but with a huge test battery on OBS (https://openbuildservice.org/). Snapshots on upgrade make the thought of breakage (haven't had any) tolerable.
It is less user friendly/more secure OOTB, in contrast to Ubuntu/Mint, still worth it IMHO, after reading the horror stories about Docker and UFW on Ubuntu.
Getting KDE Connect (Killer feature IMO) to work needed fiddling with the firewall, but while annoying at the time in retrospect it leaves a good impression.
Podman is availlable without jumping through hoops, and nearly everthing i need is availlable via opi (think AUR but mostly official, directly from OBS), if it is not in Tumbleweed allready.
Most new users aren't bootstrapping an Arch installation from scratch. I just installed Manjaro a few days ago and it, along with Kubuntu, both ship with Dolphin by default, as does KDE Neon.
In my experience, Plasma Desktop runs well even on old hardware and uses about the same amount of memory as Xfce.
It's rough around the edges, but have you seen Windows? It's almost hilarious how any component can have UI elements from any of its iterations of the past two decades.
And ask someone who isn't familiar with it to find a setting. It's a hot mess. Just a hot mess a majority of people are used to.
One problem I do have with Plasma (though this may just be how arch packages it) - Dolphin, the native plasma file manager, is not included by default. Yes, I know it's not much of an imposition to do pacman -Sy dolphin but a file manager seems like an integral part of the system to me worthy of being included in the base package for a desktop environment.
Little choices like that affect new user adoption.