I don't think they would give windows 9 to XP users because windows 9 probably won't work very well on machines currently running XP, making the free upgrade pointless since the user would have to upgrade the machine in order to utilize windows 9. If they were to buy new machines, they would get whatever windows is out as part of the new buy anyways making the free windows 9 moot.
Now, windows 7 users on the other hand are different but I don't believe Microsoft would give these users a free upgrade either. They would lose far too much money. A lot of people didn't upgrade because of metro in windows 8, I know I didn't. I also won't move to windows 9 if it has the same UI, free or not. However, if windows 9 is done right then I would think people would want to buy it, so why give it away for free?