As I've traditionally been the person who decides this, I guess I should answer.
Generally we are including fewer and fewer new plugins in core, as we move towards pushing everything into the Moodle plugins database and using manual/automatic means of installing/updating plugins in any given Moodle. Soon you will be able to click a button in Moodle plugins database to install a module directly in your own Moodle, for example.
This is similar to platforms such as operating systems, web apps and mobile phones, which ship with a basic set of core functionality but make it easy for you to extend your platform in whatever direction you want.
The exceptions in my view are:
- Plugins that almost everyone in every education sector would find useful and fills a gap in Moodle without overlapping on existing functionality much (eg Book).
- Plugins that are small and optional but flesh-out a list of options, such as new authentication plugins or grading methods.
Maintenance is always a concern. Moodle Pty Ltd has resources for maintaining our core code but they have a limit and I have to be careful not to stretch us too thin. We love volunteers but the fact is they do often move on after some time.