There is a reason to this "free upgrade" business, which will become apparent shortly after the OS launch. No one gives away something for free or offer additional features for nothing...
If you look at Microsoft's strategy the "free upgrade" actually makes perfect sense and I really don't believe there's any hidden agenda here.
Microsoft's vision is to provide one OS and body of applications which will work across devices from small touch screen tablets/ultra portables to 5 display workstations. They are desperate to get into the tablet space and have been since Windows 8, but Win8 failed miserably because they lacked the app ecosystem to make it a compelling option in the tablet space AND they wrecked the experience for desktop users.
With universal apps - programs which dynamically adjust their UI and input options to optimise themselves for whatever type of device they're opened on - it seems (IMHO) that Microsoft finally have a good technical solution to providing a good user experience with the one OS on a tablet/hybrid/laptop/desktop... that's what they've been trying to do since Win8.
The problem they face is that they NEED developer buy in. Providing a good platform with universal apps will be irrelevant if most developers don't take the effort to leverage the functionality. AND, how many developers are going to bother investing the resources into transferring their programs into universal apps if only a small fraction of their user base (their market) is using Windows 10? Microsoft need two things: 1) to make it as attractive and easy as possible for developers to get on board their vision for "universal apps" and 2) very high and very fast take up of Windows 10 to justify developer investment in universal apps.
The Microsoft Build 2015 conference back in late April was all about #1. Offering a free Win10 upgrade for a limited time is all about #2.
Remember that from now on Windows Licenses will be tied to the device (much like an OEM license). So in 12 months time if you want to go and buy a tablet or hybrid which will hopefully offer a pretty good experience under Windows 10 with its host of fantastic universal apps (this is Micorosft's hope), you'll be paying Microsoft for a licence... and this is where they hope to make their money back. They're playing the long game and I for one think it's a pretty sensible move.