Using root taxonomy (e.g. [term:root]) does not return the term if 'term' itself is a root (i.e it has no children)? why?

Here is patch. Added possibility to use real root. Just insert real-root token instead root

It sounds like you want a token like [term:root-or-current] that has logic depending if the term itself is a root term or not. You can easily create custom tokens via the core token APIs (hook_token_info() and hook_tokens(), or via the Custom tokens module). I cannot change the behavior of the [term:root] token to fit this need.

Here is an implementation of the said hooks to add this [term:root-or-current] token: http://dropbucket.org/node/1535

In my opinion this should be the default behavior of the [term:root] token. I would be glad if this get into future version, where backwards compatibility can be broken.

Or - as a less philosophical solution - add this extra token to the token module.

Both patches won't fix this issue...

To implement the described custom token in Drupal 8, use the following in your custom module:

use Drupal\Core\Render\BubbleableMetadata;

/**
 * Implements hook_token_info().
 */
function custom_changes_token_info() {
    $info = array();
    $info['tokens']['term']['root-or-current'] = array(
        'name' => t('Root or Current Term'),
        'description' => t("The root term of the taxonomy term unless the term itself is the root."),
        'type' => 'term',
    );
    
    return $info;
}

/**
 * Implements hook_tokens().
 */
function custom_changes_tokens($type, array $tokens, array $data = array(), array $options = array(), BubbleableMetadata $bubbleable_metadata) {
    if ($type == 'term' && !empty($data['term'])) {

$term = $data['term'];

        foreach ($tokens as $name => $original) {
            switch ($name) {
                case 'root-or-current':
                    $storage = \Drupal::service('entity_type.manager')->getStorage('taxonomy_term');
                    $parents = $storage ->loadAllParents($term->id());
                    $root_term = end($parents);
                    if ($root_term->id() != $term->id()) {
                        $replacements[$original] = $root_term->label();
                    }
                    else {
                        $replacements[$original] = $term->label();
                    }
                    break;
            }
        } 
    }
    
    return $replacements;
}

I was surprised as I discovered that [term:root] is only set if the current term is not a root term. I would like to see that behavior implemented, but ss of #2, it may not get fixed. If that is still the case, at least it should be mentioned in the token description, that the token would be empty if term itself is a root term.

It would be the same for other root tokens like the the menu link's root.

BTW: The code in #3 and #5 will work only for the term name/lable, but not for chaining, as it misses following code (see D7 patch):

    if ($root_tokens = token_find_with_prefix($tokens, 'root-or-current')) {
      $parents = taxonomy_get_parents_all($term->tid);
      $root_term = end($parents);
      $replacements += token_generate('term', $root_tokens, array('term' => $root_term), $options);
    }