I don't want to get a reputation of either a 'parser' or an 'apologist' (although I might consider 'shill' if the money were good enough ), but I think the SO benefit can reasonably be deemed lifetime as well.

Your link states
Annual Companion Premier status

As a Million Miler you may invite your spouse or significant other to share your current Premier status. This membership is non-transferable and will automatically renew each year unless you let us know otherwise.

If you couple that with the granting of "lifetime Premier status to all qualifying members," isn't it reasonable to read the "Annual" nature of the companion benefit as meaning only that it is changeable between beneficiaries annually at your option, while persisting in its tenor for your lifetime?