But I strongly suspect after a year Microsoft will extend the upgrade indefinitely. The whole year limit only exists to accelerate Windows 10 upgrades (to add urgency to it).

It doesn't make sense for Microsoft to end the free upgrade program after that point. They've already sacrificed most of the private licence revenue by the majority of the people buying them already upgrading, so the remaining trickle after the year won't be significant.

But I might be wrong. They did end the Windows 8 $25 upgrade after a year for real. So we'll have to wait and see.