Thank you for the feedback. Changing the default setting is under review by the feature team, and your feedback is very useful as they are evaluating.
This feature request is working its way through the backlog queue. No new details are available yet.
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Right now, I cannot use document library (add SharePoint tab) from the root SharePoint site (https://organizationname.sharepoint.com)
I opened a ticket with the support team and they tell me "unfortunately the feature of enabling SharePoint root site collection in Microsoft Teams cannot be enable; this is something that cannot be modified by design."