Aiden2k13 :
Dogsnake :
As of now Win 10 will be offered free as an upgrade for 1 year from it's release to current legit users of Win 7/8/8.1 (http://windows.microsoft.com/en-us/windows-10/about). After the year you might expect to have to pay for the software if you want it. The link I gave you also makes clear that the upgrade will be "patched" and maintained for the life of the software and system it is installed on.
Hmm. So its like they are giving 1 year of full free trial . Good way for business. I would have bought Win 10 regardless of this one year thing because I am satisfied with my legit Win 8.1 Pro and Win 10 is more good as per my friends who are using the preview version.
The language used is misleading. It is not a free trial; if you upgrade in that first year you get Windows 10 free for the life of the PC.
You would only have to pay if you hadn't already upgraded during that year period. That is if they decide to introduce a price after that.
Regarding your question about how MS will make money - one idea is that they will want to get everyone onto the Windows store so they can sell them further software but also I would guess that most copies of Windows sold are for new PCs rather than consumers buying upgrades so they will still make plenty from the OS.