These days you don't get a Office copy to use, you get 1 year free...
It's not just a Windows phenomenon. I'm sure 90% of Android users would never pay for and actively install an Android version upgrade. Even OS X upgrades are now free, supposedly for the same reason.
So if Microsoft makes very little money on consumer version upgrades anyway, they could just as well let everyone use the latest version for free and make life a lot easier for themselves and for millions of developers.
It's a very welcome move that is long overdue.
"Once a device is upgraded to Windows 10, we'll be keeping it current for the supported lifetime for the device."
I'm not sure what they mean by "lifetime for the device", but I think it means you have a year to decide to upgrade to Windows 8, and then it'll be supported for as long as your device is working.
[0] http://d35lb3dl296zwu.cloudfront.net/uploads/photo/image/191...
This looks to be an embrace and extend :) of their own idea.... Now free for a whole year.
Obviously more details once it is closer to release but sounds great so far.
So "Yes, for good reason" is the answer to your question.