No delete link when editing a menu, only reset links

Problem/Motivation

Steps to reproduce:
Install, add a link to Tools menu and go to admin/structure/menu/manage/tools.
There's no link to delete the menu link. There's a reset link, but following that leads to an error:

Recoverable fatal error: Argument 1 passed to Drupal\Core\Menu\MenuTreeStorage::save() must be of the type array, null given, called in /home/sb56c5d1abc73ea6/www/core/lib/Drupal/Core/Menu/MenuLinkManager.php on line 405 and defined in Drupal\Core\Menu\MenuTreeStorage->save() (line 256 of /home/sb56c5d1abc73ea6/www/core/lib/Drupal/Core/Menu/MenuTreeStorage.php).

Notice: Undefined index: menu_link_content:749ebca4-77db-43af-9706-06d286d0479c in Drupal\Core\Menu\MenuLinkManager->resetInstance() (line 403 of /home/sb56c5d1abc73ea6/www/core/lib/Drupal/Core/Menu/MenuLinkManager.php).

Discovered in #2313263: Page not found after adding, editing or deleting a menu link:
In MenuForm.php we have:

        // Links can either be reset or deleted, not both.
        if ($link->isResettable()) {
          $operations['reset'] = array(
            'title' => $this->t('Reset'),
            'url' => Url::fromRoute('menu_ui.link_reset', ['menu_link_plugin' => $link->getPluginId()]),
          );
        }
        elseif ($delete_link = $link->getDeleteRoute()) {

Where isResettable() is:

interface MenuLinkInterface extends PluginInspectionInterface, DerivativeInspectionInterface {
...
  /**
   * Returns whether this link can be reset.
   *
   * In general, only links that store overrides using the
   * menu_link.static.overrides service should return TRUE for this method.
   *
   * @return bool
   *   TRUE if it can be reset, FALSE otherwise.
   */
  public function isResettable();

In MenuLinkBase.php:

abstract class MenuLinkBase extends PluginBase implements MenuLinkInterface {
...
  /**
   * {@inheritdoc}
   */
  public function isResettable() {
    return AccessResult::forbidden();
  }

This makes that if ($link->isResettable()) above always true.

Another usage sample from MenuLinkResetForm.php:

  /**
   * Checks access based on whether the link can be reset.
   *
   * @param \Drupal\Core\Menu\MenuLinkInterface $menu_link_plugin
   *   The menu link plugin being checked.
   *
   * @return \Drupal\Core\Access\AccessResultInterface
   *   The access result.
   */
  public function linkIsResettable(MenuLinkInterface $menu_link_plugin) {
    return AccessResult::allowedIf($menu_link_plugin->isResettable())->setCacheable(FALSE);
  }

Where allowedIf() is:

abstract class AccessResult implements AccessResultInterface, CacheableInterface {
...
  /**
   * Creates an allowed or neutral access result.
   *
   * @param bool $condition
   *   The condition to evaluate.
   *
   * @return \Drupal\Core\Access\AccessResult
   *   If $condition is TRUE, isAllowed() will be TRUE, otherwise isNeutral()
   *   will be TRUE.
   */
  public static function allowedIf($condition) {
    return $condition ? static::allowed() : static::neutral();
  }

Unless I'm missing something this turns AccessResult::forbidden() to AccessResult::allowed().

Proposed resolution

Make isResettable() return bool
or
Rename isResettable() to getSomething() or checkResetAccess() and fix usages?

Remaining tasks

User interface changes

API changes

Beta phase evaluation