And why rewrite and push a new inferior version to users who already paid for the "good version". This is a tools that millions of their users rely on to do their job every day, it's extremely unprofessional.
That might not be enough reason in and of itself to throw out the whole application, but I sure won't miss that one part of Outlook.
They could, if they want, use WebView2 - embedding MS Edge.
I know at least one CEO who will absolutely explode if they fuck up his Outlook. He probably have 20 years of business correspondences in there. Is that a good idea? Probably not, but he does. If Microsoft mess up Outlook, then there's very little reason for his company to not look at Google and Gmail, and he probably will just because he'll feel like Microsoft screwed him over.
It was supposed to be Outlook 95 and ship with Office 95, but the app was late.
Remember that adding an account to new outlook means all your email gets uploaded to O365 servers. It no longer interacts locally. That includes external services like Gmail etc.