Change the name of the ‘Default Template’

There is a filter for this since version 4.1; cf. https://github.com/WordPress/WordPress/commit/7cdbac53e8497b346d1009375d36586fb6e5197c

You can now use:

add_filter('default_page_template_title', function() {
    return __('My default template name', 'your_text_domain');
});