I don't think this is nearly as crazy as you may think at first glance
Imagine if it was just a hidden (special) folder in an Exchange mailbox.
Voila, you already have a well-known and widely implemented and tested message syncing solution both for content and status (read/unread)
I assume Windows Phone worked the same way with its text message backup. When you'd set up a new phone it would take a while for your Microsoft account to finish syncing during which new messages would trickle into the Messaging app in real time. In fact if your old phone was still on WiFi new messages would show up on both. Still more advanced 15(?!) years ago than my Android today
When you dig it up, it is totally crazy and the total shit that we could expect.
Nothing works really well nowadays with exchange (classic, new, web, ...) or Teams.
It is a complex layer based on sharepoint, that was not designed for that, because OneDrive is so bad that they have absolutely no way to manage a proper sharing of files between multiple persons, and so even less between teams and orgs.
I the nice things like that, if someone gives you access to their Teams but you don't have a "storage" license on their domain, you will be able to exchange messages but a lot of things will not work without explicit clear error.
And especially sending or receiving a file, image or anything in a chat conversation.
Imagine if it was just a hidden (special) folder in an Exchange mailbox.
Voila, you already have a well-known and widely implemented and tested message syncing solution both for content and status (read/unread)
I assume Windows Phone worked the same way with its text message backup. When you'd set up a new phone it would take a while for your Microsoft account to finish syncing during which new messages would trickle into the Messaging app in real time. In fact if your old phone was still on WiFi new messages would show up on both. Still more advanced 15(?!) years ago than my Android today