Thanks for posting the link up, interesting read and relevant to many things I am working on at the moment.
however is not what they are talking about when they are refering to legacy systems; at least they are not specifically talking about Windows. Legacy systems could be anything. It could even be an app written fairly recently in Java and hosted on RHEL.
Legacy systems are just generally the systems that are no longer under active development and a company has no desire to go through another development cycle to move them to a new platform, they are operationally supported and might still recieve some bug-fixies but that is pretty much it. Sometimes however, a business decides that managing their own data-centre and hardware is no longer ciost effective and they want to migrate everything to a fully virtualised private cloud, a public cloud, or maybe into PaaS solution. If there is a need to some can easily be migrated and others not. Hence the need to maybe keep some where they are until they are ready to be switched off and set up a hybrid solution in the interim.
Where I work 'legacy' systems are a problem, they includes thousands of servers, running all sorts of Unix, Linux and Windows Operating Systems, many with proprietary database engines (Oracle, SQL Server, DB2) and many proprietary applications - SAP and stuff written in all sorts of langauges that we don't necessarily have source code for (it's a mess thanks to years of out-sourcing, which is now changing)
Sometimes, these are easy to re-platform - I can do a Physical to Virtual migration and shove the virttualised server into Azure, Softlayer, or an on-prem private cloud. It's still a legacy app, but now nested inside a modern platform with modern hardware. I can forget about it for a while...
Then there are applications that are not legacy and the business want to move them to a cloud platform for global scale-out - but the logistics of doing so are not easy. Maybe the application developers were lazy and assumed that the application database would always be on the same server as the application so wrote a chatty application that cannot tollerate much network latency. Maybe their chioce of database and decision to write a stateful web app means it's going to be a pig to scale-it-out without a serious re-write. Suddenly this application is now considered legacy as its more efficient to start from scratch top re-write it to use all Azure PaaS services, or maybe to sit it inside Docker containers and so on.
The term legacy is very subjective and can be applied to any application stack/language/operating system. Nice try to use it in another effort to show how irrelevant Windows can now be :-)
If I really wanted to (and I would really need to want to, because it might not make much sense) I could build an entirely modern infrastructure platform - a private cloud. That would run 100% Open Source applications inside Linux VM's using Linux hosted containers and yet use Microsoft owned software for 100% of the Private Cloud infrastructure. I could also deploy the same applications into Azure, and link the two to have hybrid cloud/on-prem deployments. Which is actually pretty cool - but use the new German Azure datacenters, because Microsoft doesn't own them and the US Government cannot force data disclosure ;-)
My Hipster developers would not care - they only see Linux & Open Source at their layer. My 'legacy' Windows Admins who (are arguably brain-washed) look after the infrasture and haven't had time or inclination to learn about Linux and Open-Source will also be happy....
This is now my life. I work for one of the biggest European based global corporations (inside top 100 for EU). Recently we (IT services) have made the case that Open-Source is the future. The leadership have bought into this and enthusastically aprroved. We are also a big Microsoft customer, and this won't change... it's still a good partnership - for us.
[EDIT - Apologies for spelling/grammer errors, there are probably quite a few]