As Outlook.com stands today it could not be packaged as a PWA and replace the Windows Mail app. Outlook.com is an email service, but the mail app is an email client. There is no way to connect multiple email accounts through Outlook.com similar to how it is done in Windows Mail.
Microsoft could obviously update Outlook.com to become a mail client which works to display and manage any email service, but right now that is not how the web app works.
Am I missing something here? What do you think, should Microsoft convert Outlook.com into a universal web based email client?