moodle.scss and default.scss are different files with different purposes.
moodle.scss is a file which calls the bootstrap ones, default.scss is a preset that actually provides the look and feel (in boost it is fairly plain, but in fordson it started out as the flatly bootswatch and in waxed for example, it started out as the Cerulean bootswatch)
default.scss is a preset file which calls moodle.scss it does not replace it - and that is common with all the standard preset files, including the presets in Fordson, waxed and those on moodle.net
You'll find a line in all of them...
// Import everything.
One way to achieve what you are suggesting would be for the child themes to implement
as a single file call instead of each of the individual sections - but that moodle.scss file itself is required in each child theme or the standard presets wont work.
For me, personally (in Waxed), that decision was a hold over from when I developed Flexibase and was overriding some parts of the upstream code and I found it helpful to be able to identify the separate components, but as you say, with a complete developed theme it would definitely be possible to bring that list back down to simply importing the boost/scss/moodle.scss file rather than all the individual ones. It has the downside though of preventing Chris' work around for the page refresh times - you wouldn't be able to comment out just the fontawesome scss files in the child theme then, only in Boost itself.
That could simplify the child theme moodle.scss (in, for example, Waxed) down to something like:
// Import core scss from Boost.
// Import Waxed theme specific styles (not preset).
Hope that helps,