"If you do not claim this offer, you will need to purchase additional storage or remove some of your files. Otherwise, 90 days after you receive your first notice, your account will become read-only."

Examples for things that will break once your account is read-only:

* Any program that modifies files on open (Word, Excel, others)

* Any program or device that would sync to a cloud folder on OneDrive

* Any existing workflow that relied on OneDrive files for easy access and/or sharing

Have you ever mounted a filesystem read-only? You should try it and then come back and tell us that people have plenty of time. They might not lose their data immediately, but virtually anything that touches OneDrive will break once it becomes read-only.