There ya go; they would complain at the prospect of releasing their source code; which would mean they would chose not to GPL it. At this point, I can guarantee they would find a means to create a new cash cow if they entered into it. It wasn't that long ago that SCO was it? or was it another Linux distro entered into a scuffle over this. Course users were like "pfft, there's other Linux distros".
Microsoft wouldn't start there; but rather start with something along the lines of working in software compatibility for all the Windows software out there (something that would need to happen if Windows got abandoned), along with making it more user friendly. Which would be a smart thing to do from the standpoint of market share, but also necessary if it could become a viable replacement in the home user market space. Until then, they could bide their time and be patient until the time would be right for what had gone before. History also serves here; there was a time where MS-DOS wasn't the only DOS and there was talk even of OS/2 becoming a major OS, but the rest was history.
On the server side of things; pfft, who cares if the web server can run MS Office, AO-Hell internet software, or whatever. However granny would want to play online cribbage, or whatever card game. Now, if they chose not to GPL, but keep the code necessary for this compatibility closed source and can get by without a GPL; they can make it as proprietary as possible. And when it comes to dominate, bam then dig their heals in for extra revenue.
And yet as all this is happening, many will also remember what happened on the system builder's side of things. Outside the Dells, Gateways, and other such tier-1 OEMs, there just wasn't as much money in it. And their better bet was to move away from revenue based soley on product, and move to one where revenue was based more upon providing support and services. Networking was also large in terms of demand around this time (end of the dot.com bubble), and before many of the jobs ended up over-seas so people calling support got people who couldn't speak English on the other end of the phone.
In fact, come to think of it; I can almost imagine there would be money to be had for a business located here in the US, which people could call other then companies own support lines, on grounds "I get someone who can actually speak my language, on the other end of the phone". ATM, it's more the smaller independents in terms of computer shops who moved into that area, as system's building became less profitable for them, but many people in a jam couldn't get far speaking to someone in Bombay who didn't understand a word they said, and were pretty much reading from a script anyhow. My father used to call me 2,000 miles away, exasperated after spending 45 mins on the phone with a "help", and describing just this situation.