Currently, mediawiki.widgets is one module that loads every widget that exist in MediaWiki. That makes it easy to use Widgets (simply by loading mediawiki.widgets), but it is (or can be, in future) a real performance problem. If you only want, as an example, to use the UserInputWidget, you still need to load mediawiki.widgets, which loads all other widgets, too (currently 33,4KB compressed), while 90% of the code isn't needed. That is, maybe, not a big problem now (33,4KB sounds very tiny compared with other modules), but if we create more and more wiget that will be a problem sometimes. And 33,4KB more or less on mobile (if we want to have widgets in mobile, too, which I assume, the module has the mobile target) is a big deal, if you don't need 30KB of it.
This task should be for dicsussions and a solution how we want to handle that. Maybe it's a good idea to have one module for each widget or another better separation (some widgets in one module), but all widgets in one module doesn't seem to be the right solution from my point of view.