Alter existing routes doesn't work I'm working on a D8 project trying to add custom access rules to some pages. 

I want to restrict access to specific users based on their id.

I checked this and seems easy but I can't make it work!

I want to change the access for the route 


  entity.webform_submission.canonical
  /admin/structure/webform/manage/{webform}/submission/{webform_submission}


But as I didn't manage to make it work I tried an easier example 


  contact.site_page
  /contact


(Which also doesn't work)

So I created my routing file under my_module/src/Routing folder :

namespace Drupal\my_module\Routing;

use Drupal\Core\Routing\RouteSubscriberBase;
use Symfony\Component\Routing\RouteCollection;

/**
 * Listens to the dynamic route events.
 */
class RouteSubscriber extends RouteSubscriberBase {

  /**
   * {@inheritdoc}
   */
  protected function alterRoutes(RouteCollection $collection) {
    if ($route_item = $collection-&gt;get('contact.site_page')) {
      $requirements = $route_item-&gt;getRequirements();
      unset($requirements['_permission']);
      $route_item-&gt;setRequirement('_access', 'FALSE');
    }
  }
}


After that I added the service file in my_module's root folder:

services:
  my_module.route_subscriber:
    class: Drupal\my_module\Routing\RouteSubscriber
    tags:
      - { name: event_subscriber }


But I don't get Access denied when I visit those pages.

Also, is there any way to test it by using dsm or var_dump?

Any idea/help/suggestion is valuable!

Thank you in advance.
you need to enable kint() to get the routs but you need to make sure the permissions for using kint() is correct.
and you can add your debugging into alterRouts()