I'm pretty sure Calibri is included with Windows since Vista (along with Segoe UI)- they wouldn't just simply stop bundling it. But if that would happen for whatever reasons then probably Office would ask user if missing font should be downloaded and installed on the system, to provide "best experience" in viewing and working with the document that uses it.
No particularly interested in MS Word or this font though I guess it's an improvement over the Calibri one, but it's kind of funny it's already 2023 and they seem to be more more worried about the default font than fixing once and for all the issues with unexpected layout breaks - unless the memes about it make for advertising
I’m guessing Microsoft has different teams dedicated to different tasks. They can probably work on layout problems and change the default font simultaneously.
They'd rather people buy the new version of Office than fix older versions. It's like a kind of indirect planned obsolescence.
Office has options to embed fonts in the document file since I could remember.