Ancient has some good points.
About forking. It seems that for Linux, forking is the nature of the beast. Since folks have tried to make their own open source version of Linux, there have been a number of interpretations as to just what Linux is, and it got so bad that standards committees started emerging (like SUS), and bare minimum compatibility standards (like POSIX). So how exactly can the open source Linux variants balance the future with Unix's past and still keep the pace of modern hardware? And since something either hasn't been done in Unix, or because Unix proper is still mostly closed source, that creates room for differences in interpretation when it comes to adding new features. So different teams add the new features differently or create conflicting standards.
As for Windows, nobody has ever created a Windows-like OS other than Windows. There are kids playing with their own Windows kernel, but most of those are grabbed up by Microsoft, and the rest are restrained to keeping their kernel-making a hobby. Due to the legalities, they can never make their own competing Windows version, at least not in the US. So the only team worth mentioning in this is the ReactOS team. And there have been "fork" attempts, but those are mostly just vanity releases. Like if I wanted to release my own, call it PurplOS, etc., chances are, someone doing that would likely only change the name and theming, maybe change some of the registry defaults or bundle additional software. At least one removed some of the sanity checks to get something they used to run or something. Those who create vanity forks tend not to maintain their "project" for long. It is all about stroking the person's ego or getting attention, and when that gets old, they abandon their project, maybe let their site registration lapse if they even started a site, and didn't just have a repository page only.
So the Windows-like alternative world is rather sane, since ReactOS is the only one of note, currently. While I wish GreenTea OS or whoever success, I just hope they are following the laws and coding it cleanly without using tainted sources. I would hate for a project like that to pull ours down and destroy the playing field for everyone.
A bone I have to pick is that I don't recall the name Linux ever being contested. Linux sounds vaguely similar to Unix. And while there were Linux suits, like SCO v. Novell, I haven't found any over the name. Yet, someone tried to make a Linux that behaved a bit like Windows and call it Lindows. And guess who sued? As a result, they became Linspire. The funniest legal action by Microsoft was the one where a guy named Mike Rowe decided to create his own software company. Guess what he called it? Mike Rowe Soft. And he was forced to agree not to use that name.
Now, if some decent ReactOS forks were to pop up, and with new developers, that could become beneficial, but I see little interest in that.