Remove the default Opensolr connector

Then what is the use of the credential config in admin/config/search/search-api/opensolr when using basic auth?

The Opensolr configuration form is required for API calls between Drupal and Opensolr. Without it, Drupal would not be able to list the available indexes or do similar API functionality.
https://opensolr.com/faq/view/api-index-management

On the other hand, the Basic Auth at index level adds an extra layer of security when searching for content.

Ok I understand. But it was quiet easy to not use the module at the end and copy/past the needed configuration.

Sure, people are free to go for any of the two modules, depending on their own comfort and usage.

Fixed and will be available starting with version 2.2.0.
One must run the updates when downloading the new version and export the changes.
All potential existing servers that are still using the deprecated opensolr connector will be converted to basic_auth_opensolr.

Automatically closed - issue fixed for 2 weeks with no activity.