I build Managed Packages (with namespaces) for my ISV customers. I use external libraries and Open-Source frameworks like [fflib][1] or [apex-domainbuilder][2] and find dependency management gets more cumbersome the more external libs I use. 

I decided to use `Unlocked Packages` to bundle and manage such dependencies better. Here are the `sfdx-project.json` files for the app and for a sample dependency.

**`sfdx-project.json` of the Managed Package (Customer DevHub)**

    {
        "namespace": "NAMESPACE",
        "sfdcLoginUrl": "https://login.salesforce.com",
        "sourceApiVersion": "48.0",
        "packageDirectories": [{
            "path": "force-app",
            "versionNumber": "1.1.0.NEXT",
            "package": "myManagedPackage",
            "dependencies": [
                {
                "package": "noNsUnlockedDependency@LATEST",
                }
            ]
        }],
        "packageAliases": {
            "noNsUnlockedDependency": "0Ho4A000000fxXpSAI",
            "noNsUnlockedDependency@LATEST": "04t4A000000onRXQAI",
        }
    }

**`sfdx-project.json` of the Required Library (Developer Devhub XYZ)**

    {
        "packageDirectories": [{
            "path": "force-app",
            "default": true,
            "package": "apex-domainbuilder",
            "versionName": "Summer '20",
            "versionNumber": "1.1.0.NEXT"
        }],
        "namespace": "",
        "sfdcLoginUrl": "https://login.salesforce.com",
        "sourceApiVersion": "48.0",
        "packageAliases": {
        "noNsUnlockedDependency": "0Ho4A000000fxXpSAI",
        "[email protected]": "04t4A000000onRXQAI"
        }
    }

As the sample dependency is created from another DevHub and/or doesnt have a namespace I can't install them.

> ERROR running force:package:install:  You cannot install an unlocked
> package without a namespace into an org with a namespace.

 1. Why is that a requirement? 
 2. Is it because the libraries use a different DevHub (the one of our company) than the Managed package (the one of the customer)?
 3. I don't want (I read [here that this has some problems and limitations][3]) or can't (because they are not my repos) add namespaces. 
 4. What will I win and lose when adding namespace to the dependencies?


  [1]: https://github.com/apex-enterprise-patterns/fflib-apex-common
  [2]: https://github.com/rsoesemann/apex-domainbuilder
  [3]: https://sfdc-db-gmail.github.io/unlocked-packages/faq-unlocked-pkgs.html#namespace