The default theme cannot be deleted because, even if you have several other custom themes installed, these themes almost certainly still depend on files in the default theme. If you were to delete the SMF default theme, none of the other themes would work. In fact, you should probably avoid making any changes at all in the default theme directory, because doing so might cause some custom themes not to work properly, or make it difficult to install modifications.

If you do not want your members and guests to see the default theme, you can set a different forum default theme. You can even stop them from seeing the default theme at all, using Themes and Layout >> Manage and Install.

If a problem in a custom theme prevents you from using the forum, you will always have the SMF default theme to fall back on. Just use ?theme=1 in your URL. If you install a mod that "breaks" the default theme, you either have to get a working set of files back again or use repair_settings.php. For more on this, please see A mod broke my forum.