They only placed a one year time limit on it in order to increase early adoption. They will no doubt "extend" the period right before its end due to "overwhelming popularity" and then extend it indefinitely after the buzz from the first extension has worn off.
For Microsoft's current strategy it doesn't make sense to leave even a single user on Windows 7, 8, or 8.1. They need everyone on Windows 10, and further still the number of people who, missing the free period, will pay to upgrade to 10 is going to be miniscule.
I might be wrong, but I don't think I am. It is just more compatible with Microsoft's strategy.