What problem does this address?

If themes ship with templates, it is impossible for users to opt out of them. For example if a block theme contains a front-page.html file then users have no way to stop this template from being used on the front of their site.

What is your proposed solution?

There should be a mechanism whereby users can opt out of a default theme template.

Here's just a rough design to get the ball rolling:

1. Adds a "deactivate" option to the template
2. Show inactive templates as grey.