IMO Buttons/Sliders/Radio Groups and that kind of UI component are already achievable relatively easily with existing Eventsheet features (Families/Hierarchy/Custom Actions)
While it would be great to have those "Components" built-in in C3, IMO this isn't the real pain we're facing with UI right now. It could bloat the engine with a bunch of additional Plugins/Behaviors and each person might want their Sliders/Buttons to act very differently. So Component would probably need too many features and parameters to cover everyone's need and might even be disappointing in the end, and dismissed in favor of "custom Components" created with existing features.
If those potential "Built-in Components" were Plugin, it would be the same pain as today as there is not Multi-Plugin support for Families in C3 for example
The biggest pain is manipulating the position (transform) of all UI Objects with auto-layouting which is almost impossible to achieve even with Families as Text and Sprite for example are different Plugin and can't share common logic. (it would be the same issue with Button/Sliders)
This is why Hierarchy based autolayouting would solve this as it works with any world object (+ easy picking benefit and intuitive display in a potential Hierarchy view)
Hierarchy based autolayouting + a "Selectable" behavior would help us to make those Slider/Button/Grid/Scrolllist "Components" ourselves