As I understood it, it is only free for previous Pixel owners until they buy a new Pixel. So it is free for the lifetime of the device, not the users lifetime.
Isn't that how most people would have understood the offer anyways? Why should Google store the photos from e.g. your new OnePlus for free just because you owned a Pixel in the past?
Presumably because they promised the service in return for the purchase of a Pixel device, not its continued use.