Using Dependency Injection within BatchService doesn't work

Problem/Motivation

As raised in #3159239: Error: Using $this when not in object context → you can't use dependency injection within BatchService as the functions that get called are used as callbacks, so you don't really have access to $this within the scope of the processMyNode or processMyNodeFinished. The entirety of the code added in that file is completely misleading. For example if I wrote the following, it wouldn't work, and you can't inject services within create/construct since they never actually get fired. There is no "service" here at all.

protected $connection;

public function __construct(Connection $connection) {
    $this->connection = $connection;
}

public static function create(ContainerInterface $container) {
  return new static(
    $container->get('database'),
  );
}
public function process($chunk, $batchId, $operation_details, &$context) {
  $context['message'] = $operation_details;
  foreach ($chunk as $instance) {
    $this->processFieldInstance($instance, $context);
  }
}

public function processFieldInstance($instance, &$context) {
  $query = $this->connection->select($instance['table'], 't')
    ->condition('t.bundle', $instance['entity_bundle'], '=')
    ->condition('t.entity_id', $instance['entity_id'], '=')
    ->condition('t.revision_id', $instance['entity_revision_id'], '=')
    ->execute();
}

Steps to reproduce

1. Try making your batch callback use $this to call another method within the BatchService class.
2. Or try using a service via dependency injection

Proposed resolution

Not really sure, research leads to #2875151: [META] Implement Batch API as a service → which if you follow the rabbit hole you need to do things like in this post, https://www.previousnext.com.au/blog/refactoring-drupal-batch-api-callba...

Remaining tasks

Discuss, and 100% don't refer to the file as a service when it is not. It's just a class, that isn't current even instantiated since the methods are called directly via call_user_func_array().