Adults can use training wheels too when first learning to ride a bike. But once they're competent, it's more expressive to ride without.
Use by the non-initiated may not be that high, but that shouldn't be surprising given that user education is a very scarce resource and it's being spent elsewhere.
Every KDE I've installed (Slackware, Mandrake, Ubuntu, Fedora), to my recollection defaulted to multiple desktops.
I'm so happy Win10 has virtual desktops as in moving from Linux to MS Windows - mandated by my workplace - that was a massive pain point.
It might not be the most visible feature for most users, but it is definitely an important one, which I would hate to see go away.