Which means they know extremely well what a costly and complex process it is.

I'm not denying that it's a complex or costly process, just lamenting that they don't care enough. Of course, the problem is compounded for me by the fact that at any given moment I end up having 3 different enterprise chat apps open on my work laptop for collaboration with outside parties.

And the result of that knowledge is a worse experience for users. However you frame it, users are losing because MS doesn’t want to shell out the cash for good UX.

Losing, but still buying in.

The users aren’t really, their bosses are, most likely because it gets bundled in a contract with Office and Exchange etc.

Outlook is switching to WebView2-based UI like this soon. As for Excel, here's its PM saying "When you look at the overall allocation of resources on the team, a huge percentage is working on web app improvements" https://twitter.com/jones206/status/1388162347059978243?s=20

But the mac version has never been the same as the Windows version.

Actually the Office team is integration more and more React components in the office programs.

Perhaps the effort/reward they get from office release does not apply to a freemium chat service that’s constantly updating?

Office is special because it's such a cash cow they can afford to make 4 different versions of it.

They don't have feature parity though. For example, power query doesn't work on mac.