I think they'd be quite happy to undercut Desktop/Mobile Windows in favour of Office. Long term Office may be the most viable product. There are already online and mobile versions that don;t require anything Windows related (they aren't feature complete yet by any means, but that will eventually change).
Even Server Windows to an extent if their other on-prem cache cow (SQL Server) does well on Linux.
Windows is not their core priority these days. The Azure platform is, and Windows is just a part of that which could be swapped out. Maybe Windows will just become a free (though obviously not Free) product, with the money coming in from Azure + SQL + Office.
Exactly my thinking. Office probably has more legs. Attempting to keep it tied to Windows in order to save Windows could end up bringing both down, and I think they know this.
Back in the bad old days, Microsoft could barely be convinced to keep a Mac port of Office going, and that was basically to keep the government at bay. But they seem to be more open now.
Obviously this would require Microsoft themselves to undercut Windows, but Office is such a cash cow for them, they might see it as worthwhile.