Currently, if you create a Template for an Element, deleting the Template from the Resources will cause the Elements using the Resource to break rather than being 'de-templated'. Although under the hood it has been reverted back to the Default Template for the Element (as visible through the Context Menu's assigned Template), in practice, it just deletes all appearance of the Element in the Page.

See attached video.

Repro Steps:

1) Create a Project including a MainPage.xaml
2) Introduce a Button to the Stage.
Observe the default appearance of the Element's built-in Template.

3) Right-Click on the Button Element, and navigate to 'Templates', 'Button', then 'Create'.

4) Create the Button Template inside of GlobalResources.xaml
The Template for the Button will then be opened within the Dummy Template Editor.

5) Exit the Template by clicking the 'Exit Scope' button in the lower-left of the application Window.

6) Navigate to the Resources Panel, and find, then delete the Button Template created in Step 3)+Step 4).

Observe the Button on-Stage collapsing into nothing.

7) Right-Click on the Button Element in the Navigator, and within the 'Templates' then 'Button' sub-context menu, observe how the Template being applied is the '[Button Default]'.
Observe how the button on-Stage doesn't reflect what the Button's default Template actually is.

Expected Results:
Removing an Element's Template from the Resources reverts it back to its Default Element Template.