Where is that info coming from? To my experience Plasma is even lighter than GNOME, or just a tiny bit more resource hungry than something truly lightweight like XFCE. It doesnt really need powerful computer.
Yeah on modern hardware, resource usage by the DE is negligable compared to what some applications use (for example web browsers, or chromium based apps).
300MB of RAM vs 400MB don't make any difference on even fairly old systems. It might make difference if you try to squeeze your modern Linux distro on 512MB. In that case you might want to just use a window manager and distro with no systemd, something truly minimal like Apline. If we're talking 4 gigs+, the desktop won't make much difference anyway. On very old computers modern web browsers will always suck no matter the OS.
Agreed. I use older computers a lot and even use a 2008 machine for hours a day because I like the screen and I like where it is in the house.
I can say that with 8 GB of RAM or more, even older CPUs are quite usable for me. Less than about 6 GB and even basic web browsing is going I get fairly painful.