Shouldn't we call it "free trial" rather than "free"? As far as I understand it, we're going to have to pay up after the first year of "trial". This goes for everyone including us who got a full Windows installation.
You understand incorrectly.
Once you have a Windows 10 key, its yours.
To make it more obvious, the price for the foist year will be $0.00, and if you "buy" it during this time for the low, low price of $0.00 then you get a key that doesn't expire.
If you buy it after that year the price will be more than $0.00 (let's say $x, where x is not zero) and you will get a key that doesn't expire.
What will almost certainly happen though is that the period where it is available for free will be extended indefinitely. It's clear they want to push adoption of the Windows ecosystem to make it more homogenous and the best way to do that rapidly is to make it look like the window for the free upgrade is limited. I suspect it will be free forevermore, but they want a rapid uptake of users rather than a slow trickle.