From a business standpoint, they trust Office. End of story. Microsoft has to fail for that to end. When I write reports for business analyst, the most consistent way to get them angry is to not include export to Excel. "oh, you wrote the report I asked you for, where is the export to excel? Screw you then, put that in and I will talk to you." Those people live in Excel, Word, Powerpoint, and Outlook. And there is no force on the face of the Earth that will compel them elsewhere.
Data Scientists might like R or Python with Machine learning...business worker don't need any of that crap, Excel does everything their imaginations would ever want.
Second point: if something goes wrong, what a business manager wants is a throat to choke. Who to I call up and swear to when something goes really wrong. They want to know that someone is getting fired over their outage. Microsoft gives them that (not the firing part, but they know how to play along). Better if it is a big company that they know has dealt with "big" problems like theirs before. Open Source? You mean some guy in his mom's basement? Sure, we can go after his lunch money, but that is not nearly exciting enough.
TL:DR: Unless Microsoft screws themselves and does something really wrong, no one else really has a chance.