Using New Outlook on Windows for professional communication and productivity
After thinking about this for a while and working with Microsoft's copilot interfaces across their ecosystem, my current theory is that one of the reasons they have delayed the unified inbox is because copilot may not work uniformly across all the different email services that can be unified in Outlook.
In fact, it seems like you would need to replicate all the email storage across all the accounts to truly apply copilot uniformly, and even there it may yield surprising results if crossing different business domains or business and personal accounts, whereas applying it consistently and evenly across a single email account on a per provider basis would be more straightforward.