I think you underestimate how slowly things move in some places. There are still shops running COBOL on mainframes. Windows is going to have a place on servers for decades to come.
With .NET 5, Windows to Linux migrations are going to become faster.
One can only hope... Containerization on Linux is so much smoother imho. Not being able to run both win and lin containers for developers is a big issue. Linux owns this space, and it will likely continue down that path. Currently doing a couple .Net core projects with deployments to Windows and Linux.
News flash, there is more COBOL being written today that in any prior year. It's a small part of the total output, but is not decreasing in absolute terms.
What is the source?
Yes, but in the same way companies are still running XP. There is zero chance there will be new releases of Windows Server a decade from now. Microsoft itself is starting to host its services on Linux.
I don't see a Linux version of Exchange or Active Directory yet. For internal email, it's pretty much Exchange or GTFO.
OpenLDAP, Postfix/Dovecot?
And all the other stuff?
Not a fully integrated email and calendaring solution that can be set up and administrated by typical overworked corporate IT staff. Completely unacceptable.
Then what about iRedMail? Isn't that sort of an all-in-one wrapper around Postfix/Dovecot and some other stuff?
Your statement reminds me of a well known tech pundit (Cringely?) who predicted in 2012 that Windows client would die in 2017.
It's been less than a year since the last release of OS/2, I think we're a long ways away from the last release of Windows for servers.