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.
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.