Question about optional terms

See http://spec.graphql.org/October2021/#TypeDefinition. Every type definition _may_ not declare its "important" parts: _FieldsDefinition_, _UnionMemberTypes_, _EnumValuesDefinition_, _InputFieldsDefinition_. The idea is that those parts may be declared later as extensions:
```graphql
extend type Dog {
color: String
}
```

OK. Now let's look into http://spec.graphql.org/October2021/#SchemaDefinition :

Root operation types list is required. Why so? I think that this part should be made optional, guided by the fact that http://spec.graphql.org/October2021/#SchemaExtension may further add root operations.