\Drupal::config() returns immutable config. For discussions on why this is preferable see that issue.
This change also has the advantage of making it easy to use new D8 best practices for form alters. For example, constructor injection and only autoloading the minimum amount of code to perform a request. Plus form alters are one of the entry points to drupal it would be great if they looked more like the rest of D8.
Beta phase evaluation
|Issue category||Task because work arounds exist.|
|Issue priority||Major because form alters are one of the entry points to drupal and modifying configuration forms to add site specific options can be common on custom sites. It is super easy to get the config override wrong and therefore end up with overrides being stored in your configuration.|
|Disruption||Not disruptive - contrib is free to continue to implement form alters in a procedural fashion.|