Office 365 / Azure AD subscription is cheap. It comes with legal certifications that are very expensive to obtain alone and usually shareholders pressure you to get them. It has remote management capabilities and a very alive closed source plugin ecosystem that's battle tested.

How long would it take FOSS to replace this? How long would businesses and their shareholders will wait? Buying Microsoft will GUARANTEE that you'll have the certification you need in weeks, how long would FOSS would ship and PROVE that they obey a non-open "open" standard?

Will FOSS GUARANTEE cross-compatibility of the file format?

What you are proposing isn't stupid if the most important goal is independence. Most businesses don't care about independence. They PREFER dependencies they can blame and sue / settle with.