Re: It's all about the developers
Personally I think it's free because MS are desperate to show they are still relevant. If they can point to Windows 10 adoption rates and show it actually beating Windows 7 instead of languishing way behind like Windows 8 and 8.1 combined then it counters the current narrative that Windows is dying and that Microsoft are basically running a legacy platform.
The other clever master stroke was announcing you could only get the OS free for the first year, thereby causing people to rush to reserve their upgrade to ensure they don't miss out.
I also think the free upgrade queuing mechanism means that MS are in control of when people get Windows 10, so they can spread adoption out evenly and even boost it in slow months to give the appearance that Windows 10 is selling steadily and that their business model still works.
I think this is all about getting the casual users back on Windows, many of which probably use their phones or tablets for Facebook/YouTube/Twitter/Email. They don't care to buy a new PC when Windows 7 does the job, but if the upgrade is free they will decide they might as well grab it while it lasts.
That's my conspiracy theory anyway!