This is the real elephant in the room. Despite all their other flaws Microsoft has an excellent record of maintaining their software well after release. Particularly when it comes to their core products like Office or Windows. WIN32 programs or XLS files from 30 years ago still work on the latest versions.
If Microsoft didn't have this commitment, then porting Office to Linux as an experiment might be worth it. Even if there's a 10% chance it takes root, that's good revenue. But even if only a small base of users bought Office Linux it, Microsoft's reputation would require them keep maintaining it for decades.
A while ago I needed to experiment with an old file format, and I found a Linux binary of a proprietary tool from the 90s. It linked to libc version 5 (from before the Linux world adopted glibc). It would have been complicated to get it running.
I found a Windows binary of the same tool. Put it on a Win10 VM I had for other purposes. No issues.
Stable binary ABI of userland is something the Linux world doesn't do well. Win32 and COM are really good at this.
This one I don't really understand why not. Presumably Office for Linux would still bring in revenue. Just not the additional indirect revenue of a Windows license.
> (though a web interface is possible),
I am not totally familiar with the "Office365" brand and what it includes, but I get the impression some browser based stuff already works on Linux. In work accounts at former employers I have used a web-based Outlook on Linux and FreeBSD that had some "Office365" branding.
> and they won't move away from Windows-only APIs
This one I don't know why they would ever do. Win32 apps are still driving revenue and customer satisfaction for them. More, I think, than they admit to themselves. Witness they tried to kill Win32 apps in the Windows 8 cycle, and failed. Then kept trying to resurrect the idea of killing Win32, and had to walk it back every time. Surface RT wouldn't do third party Win32 apps. Neither would Windows Store initially. Then they floated a Windows 10 amd64 SKU that couldn't run third party Win32 apps. Nobody wanted this stuff. That legacy is pretty much the only thing that keeps people on Windows.