I totally agree with you @JonL!
For several (paid and free) BS themes and UI kits I tried, the templates always contain div and span elements which can be identified in code view or DOM panel of course, but don’t appear in the AC components tree.
A lot of switching from code view or DOM panel to AC is required to use and populate these templates and convert html elements to components that can be recognized and managed by AC, which makes using external custom themes very inefficient. As you mentioned here using external themes will never be a plug&play thing, but a single view tree with node filters would definitely make this a lot easier. You got my vote 