Work in #1491, incorporating @jordandukart 's suggestion.

islandora.libraries.yml

@@ -1,7 +0,0 @@
-image_media_form_defaults:
-  version: 1.x
-  js:
-    js/image_media_form_defaults.js: {}
-  dependencies:
-    - core/jquery
-    - core/jquery.once

islandora.module

@@ -334,6 +334,28 @@ function islandora_form_alter(&$form, FormStateInterface $form_state, $form_id)
   }
 }
 
+/**
+ * Implements hook_field_widget_WIDGET_TYPE_form_alter().
+ */
+function islandora_field_widget_image_image_form_alter(&$element, $form_state, $context) {
+  $element['#process'][] = 'islandora_add_default_image_alt_text';
+}
+
+/**
+ * hook_field_widget_WIDGET_TYPE_form_alter() callback.
+ */
+function islandora_add_default_image_alt_text($element, $form_state, $form) {
+  if ($element['alt']['#access']) {
+    $params = \Drupal::request()->query->all();
+    $media_of_nid = $params['edit']['field_media_of']['widget'][0]['target_id'];
+    $node = \Drupal::entityTypeManager()->getStorage('node')->load($media_of_nid);
+    $title = $node->getTitle();
+    $element['alt']['#default_value'] = $title;
+  }
+  return $element;
+}
+
+
 /**
  * Implements hook_entity_form_display_alter().
  */
@@ -486,16 +508,3 @@ function islandora_preprocess_views_view_table(&$variables) {
     }
   }
 }
-
-/**
- * Implements hook_page_attachments().
- */
-function islandora_page_attachments(array &$attachments) {
-  $current_path = \Drupal::service('path.current')->getPath();
-  $path_args = explode('/', ltrim($current_path, '/'));
-  // Need to populate the "Alt text" field using JavaScript since it is
-  // not part of the rendered form, it is added via Ajax on file upload.
-  if (count($path_args) >= 3 && $path_args[0] == 'media' && $path_args[1] == 'add' && $path_args[2] == 'image') {
-    $attachments['#attached']['library'][] = 'islandora/image_media_form_defaults';
-  }
-}

js/image_media_form_defaults.js

@@ -1,6 +0,0 @@
-(function (Drupal, $) {
-  var ImageName = $('input[id=edit-name-0-value]').val();
-  $(document).ajaxSuccess(function () {
-    $('input[data-drupal-selector=edit-field-media-image-0-alt]').val(ImageName);
-  });
-})(Drupal, jQuery);