MicrosoftI think the idea is simply the sharing and let's be honest the orginal solution was always error prone to extern users not understanding teh different between Microsoft Account and Work & School. Once the process was understood it is a good solution. For the future you are not losing B2B functions related to AAD based accounts... you have to so some extra work and maybe up your license:
Self Service Portal (GitHUB):
https://docs.microsoft.com/en-us/azure/active-directory/active-directory-b2b-self-service-portal
Or the build in features described here:
https://docs.microsoft.com/en-us/azure/active-directory/active-directory-b2b-iw-add-users
The current solution is a well known beast to longtime SharePoint Online users. We started back in the BPOS days. Lets pass another year and end of 2018 everything will seem fine as nothing changed because we adapted to the new "secure" sharing ;)
