I have another template installed and assigned as the default.

Do that template is child of cassiopea by any chance?

Good questions, but no, it's not a child of Cassiopea.

So it would appear that the problem is you cannot disable a parent template if there is a child template - even if that child template is disabled

Cassiopeia extended is disabled. No child templates. I did try disabling Cassiopeia - Default while the extended was enabled but it still says, "Disabling the parent template is not permitted."

Cassiopeia extended is a child of Cassiopeia

So it would appear that the problem is you cannot disable a parent template if there is a child template - even if that child template is disabled

Yes, so I hope Cassiopeia could be disabled in a future release.

I would also like to be able to disable the Cassiopeia template so I do not have to scroll through the list of positions to get to my default template.

I can't reproduce the issue:

also your icon looks different like in core?

@coolcat-creations but are you using the Cassiopeia_extended template or something else completely. I suspect the latter

I installed another template to test it and made it the default. Because of course the current template should not be disabled?

You dont understand the problem. The request is to be able to disable the PARENT template when you are using the CHILD template. (I know that sounds odd) so that when you are assigning a module to a position you ONLY see the positions in the CHILD template

Ok but a Child template is never guaranteed to have the Full Template. I would rather see this as some kind of Setting somewhere to define which Module positions can be shown and maybe even to whom (access levels for selections and usergroups for the display of a whole Position?).