How to add placeholder into comment form textarea?

Use the filter comment_form_fields instead of comment_form_defaults. Also, in the call to str_replace(), remove the ="" from required in both strings. So, your code will become:

function custom_comment_form_placeholders( $fields ) {
  if ( isset( $fields['comment'] ) ) {
    $fields['comment'] = str_replace(
      '<textarea id="comment" name="comment" cols="45" rows="8" maxlength="65525" required></textarea>',
      '<textarea id="comment" name="comment" cols="45" rows="8" maxlength="65525" required placeholder="Write a Comment"></textarea>',
      $fields['comment']
    );
  }
  return $fields;
}
add_filter( 'comment_form_fields', 'custom_comment_form_placeholders' );