Archivists and librarians have to think in terms of practicality: if many tools exist to read something and it’s a mainstream software product, the odds are good that they’ll be unable to use those files 50 years from now. Not certain, but good, and that matters with limited budget and ability to tell the rest of the world what format to provide things in.

This can require nuance: for example, PDF has profiles because the core format is widely supported but you could do things like embed plugin content from now-defunct vendors and they would only want the former for long-term preservation.

Especially when Office 365 shows that not even Microsoft is capable of making software which can display Office files anymore... if you have a Word file which was created or has ever been modified by the Word application, working with it through Office 365 in a browser is such a pain. I've literally had images which are impossible to delete or move in the web version, and they will absolutely render in the wrong place.

You can unzip the xlsx and read the xml inside. It’s not the worst format by far.