Let's face it, Linux has been growing since 1991 and Ubuntu is one of the more popular distributions. Ubuntu is based in Debian Linux and some failed Linux systems like Lindows/Linspire gave Ubuntu a lot of source code to help it become more user friendly. You have a whole community of open source developers contributing to Ubuntu and a ton of free and open source software that each one costs less than a bottle of water. Sure there are commercial apps to buy for Ubuntu as well.
Some feel like Apple has hit their peak in the iOS market, 500 million iOS devices sold (how many are upgrades though from older iOS devices?), and Apple had some quality control problems with their iPhone 5, Apple Maps, and iOS 6. Apple has tied the operating system to the iPhone, iPad, iPod, Macintosh forcing hardware sales of Apple branded mobile devices and Macintosh computers. Apple had placed all their eggs in one basket outsourcing to the Chinese Foconn and just now noticed how vulnerable that makes them and decided to start up a USA based factory. Apple's stock has taken a tumble despite showing profits, investors don't think Apple can grow any bigger and has hit their peak. But then investors have been proven wrong before.
Microsoft is going balls to the wall with Windows 8 and its evil twin Windows RT and that 'Surface' tablet PC with the detachable keyboard. Meanwhile PC shops and PC support sites are offering the $125 downgrade to Windows 7 to users because of how much Windows 8 sucks. Microsoft removed the Start Menu that users loved, and replaced it with the 'Start Screen' called the Modern GUI because Microsoft got sued over the Metro name, and Windows 8 GUI doesn't sound so catchy. People are confused over Windows RT for ARM based systems, it comes with a pre-installed MS-Office but won't run Legacy Intel X86/X64 Apps and it looks exactly like Windows 8. But Microsoft doesn't care they keep marketing Windows 8/RT as the net big thing and have bet most of their money on promoting it.
Why should they fear Ubuntu? Well if they upset customers too much Ubuntu can easily be installed on Windows 8 PCs and Mac OS X Macintosh systems for free. Ubuntu is being ported to smartphones and tablets and those free and open source community developers are designing a much better and user friendly GUI interface for it, plus any Linux or Ubuntu app can easily be ported to the mobile Ubuntu and there is more Ubuntu apps than iOS and Windows RT and Windows 8 Phone apps. Ubuntu stands to grow really fast once it has mobile devices running it, not only that some bold FOSS developers might port Ubuntu to those iOS, Windows RT, Windows 8 Phone devices to allow users to 'switch' to Ubuntu and keep their current devices. What will Apple do if their iOS devices are ported to Ubuntu and they can no longer receive payments to the App Store or iTunes store? The same with Microsoft. See the problem here for Apple and Microsoft? If they don't please their customers, they might jump ship and change operating systems to Ubuntu that would be a better experience for them. Did I mention that both Microsoft and Apple make their operating systems run slower on older devices to force the buying of their latest devices for a faster experience? Did I mention that Ubuntu has a lower overhead memory footprint, and a monolithic kernel that loads and unloads modules from system memory to save memory and make the OS faster?
Uh yeah, it is that serious. Almost any device can be made to run Ubuntu, it has been ported to many different platforms and many different CPU types. Unlike Apple or Microsoft, Ubuntu doesn't have a fascist policy on software applications to their software store that excludes others for 'silly reasons' and so Dotcoms based on sales from their app go bust because Apple rejected them or Microsoft rejected them, but they may find a market on Ubuntu. That means if Apple and Microsoft reject popular apps, and they end up on Ubuntu devices people will start to move away from Microsoft and Apple to the Ubuntu platform.
Steve Wozniak was once quoted as saying "Never trust a computer that you can't throw out of a window." during the 1980's computer wars with Mainframes. Perhaps the new quote in this modern age is "Never trust an operating system or software that you don't have access to 100% of the source code." Ubuntu is all open source, Ubuntu has software that is open source. While Apple supports open source projects and based Mac OS X and iOS on *BSD Unix aka Next it does not give away 100% of their source code, getting the source to Microsoft software, they'll just laugh at you.
I got tired of Windows 7 Home Premium Edition crashing on my Acer laptop and I reformatted the hard drive with Ubuntu 12.10 and never went back to Windows. I own Windows 8 licenses but discovered it was total crap and went back to Windows 7 on my PC, and then reformatted it to Ubuntu 12.10 and never went back to Windows. I own a Macbook that runs Mac OS X and I got tired of the lockups and slowdowns and crashes so I mothballed it and will only use it if I need access to xcode to develop iOS apps, as there is no other way to do so because of limits Apple placed on the app store. Ubuntu does not limit developers that way, it frees them to use any tool they need to develop for their OS. With Apple and Microsoft you have fascism, with Ubuntu you have freedom. Once the average user figures that out, I know they will look into Ubuntu and maybe other Linux distributions.