There is a sort of half-way house, whereby a second user can login
to a workstation without the first user logging out, but the same
keyboard and screen are used and the first user cannot do anything
while the second user has control. I don't know how commonly used this
is, Windows has had it for many years, but few home computers have
enough resources to do this efficiently.
Huh? Not only Windows but also GNU/Linux and Mac OS X have had this
for years. And it's not particularly taxing in resources, since most of
the applications of the "non active" users will just be sitting idle and
can be swapped out.
IOW average PCs have been able to handle this very efficiently for years.
Second and subsequent users have a poor 'user experience'.
Under Debian running Gnome, indeed, the experience is subpar: e.g. only
the first user logged in gets to enjoy audio output. This bug has been
with us since Debian/Gnome started relying on pulseaudio, IIRC.
And in my experience, the Gnome lock screen has had (and still has) all
kinds of weird lockups in these kinds of situations.
IOW, the problem is that it's not tested enough. Also, lots of
development around desktop environments have been made with brain-dead
assumptions (e.g. conflating "user" and "administrator"), forgetting
that GNU/Linux is a multi-user operating-system.
Examples go from earlier network-manager, to current pulseaudio, through
things like the "mount daemon" for USB drives.
I think the main point being made is that computers are now
sufficiently cheap that we don't have to all crowd around one machine,
that where there are two or more people in a household who use a
computer more than very occasionally, they will have their own
computers.