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It would be great if a Global Admin could gain access to extension management Page in order to be able to install and manage apps.

Often the Partner does not have an active D365BC license onto production Tenant but is Global Admin of tenant and usually it's the Partner that has to install its own extensions on behalf of the customer.
STATUS DETAILS
Declined
Ideas Administrator

Thank you for your suggestion! 
We believe that it is already possible to resolve this issue in a different way. 
Business Central partners do not require a license to login into their customers' environments (provided that they have established a reseller relationship with the customer). 

Please find more information here: 
https://docs.microsoft.com/en-us/dynamics365/business-central/dev-itpro/administration/get-started-online#connect-with-customers
https://docs.microsoft.com/en-us/dynamics365/business-central/dev-itpro/administration/delegated-admin

Partners can also assign the Extension Management permission set to any other licensed user, so they can also manage the apps. 
Local AAD administrator role in Business Central is very limited on purpose, as the actions should either be performed by Partners (w/o license) or by a licensed user.

Kind regards,
Business Central team