Hypothetical? Every major ebook platform offers annotations (and I just checked — this platform did too). MS apparently recognizes the additional inconvenience to people who have annotated, as they are offering an extra $25 to anyone who has made annotations. https://support.microsoft.com/en-us/help/4497396

I agree that this closure is not world-ending, and I think the point of the article is that if MS can do this to your ebooks, then other big tech companies could do it to your other content too.