Processes in Microsoft 365 for setting up Office apps, redeeming product keys, and activating licenses.
So this rabbit hole is much more problematic than just what you describe. We have two versions of desktop Outlook. I'll call them Classic and New.
Classic Outlook would let you add a Business Basic account as long as you had any one account added to it that was Business Standard or above or you had an apps license in some other way. Classic Outlook wasn't available to download from portal.office.com unless you had access to a Business Standard or personal account that gave you desktop client access.
As soon as you convert to New Outlook, the basic accounts are flagged as needing to log in. The login generates an error that the basic account cannot be used with New Outlook and your only option is to upgrade the license or remove the account. As you mention, this New Outlook replaces Windows 10/11 Mail. This leaves my basic clients with web mail access only, as soon as they try to upgrade Windows Mail.
The crazy thing is that you can add Gmail and Outlook.com accounts to New Outlook all day. The account that you pay Microsoft for versus the outlook.com account that you get free is the one that you cannot add to Windows 10/11 Mail (i.e. New Outlook) any longer. Everywhere I look, there is no official word from Microsoft about this gap in functionality. The idea of combining mail clients on Windows to a singular Outlook client is fantastic. The execution at the moment leaves all of their Business Basic users without a usable desktop client.
To make matters worse, sometimes New Outlook will accept a Business Basic account and let it work. I have multiple clients where this is the case. Needless to say, the policy surrounding this is not in line with customer needs and needs to change.