I'm stunned by how time & again, companies manage to fubar a running business for no reason whatsoever.

Not a clue how Glassdoor was doing before. And sure, in any field there may be competition. Business models energe & go away over time. Markets change, and successive CEOs, managers etc may have different ideas (and success) in addressing that.

But how could anyone at Glassdoor think that this policy change would not drive users away? That this would not hurt their business? (all else equal, fewer users = less profit).

If there were an upside to this policy change, sure. Weigh pros vs cons. Ask users. Do some A/B tests or whatever.

But this reads like a change with only downsides (for users, and potentially very significant) vs. little (if any) gain for the company. Why do it then?

"If it isn't broken, don't fix it", how hard is that?