Theme Customization API Changes not Saving

Well, first problem:

$wp_customize->add_setting(
    'tcx_footer_copyright_text',

echo get_theme_mod( 'tcx_footer_copyright_message' );

Those aren’t the same setting names.

Note sure why you don’t see the default value in the input field, as there’s nothing wrong there. Although you should also define the same default in the second parameter of the get_theme_mod() call.