It already supports kirby plugins (since 2014) fields and tags. I guess we could get it to support the core aswell (so no more need for the composer-custom-directory-installer.
The advantage of the composer/installers is that the plugin author controls where the package is installed - basically the author requires the composer/installers in his plugin, tells it that it is a kirby-plugin and than it is installed into the sites/plugin folder automatically once someone that wants to uses the plugin installs it via composer require someone/some-kirby-plugin - no more editing the composer.json manually.
I am aware of that. With the method I am proposing you - as the enduser - do not need to manually edit the composer.json anymore. All you would need to do to install kirby would be to run composer require kirby - thats it.
Same for Plugins, the current way to install plugins using composer I knew of was opening up the composer.json, adding the new plugin to the require object and also to the installer-paths. Now all you need to do is running composer require blankogmbh/kirby-sentry (https://github.com/blankogmbh/kirby-sentry).