Yeah, I don't think the reservation program for phones will ever be a thing, as phones will always get future updates for free, until they can no longer handle them hardware wise, just like it has always been.
I think the reservation program is just to get as many Windows users on board and ready to get 10, so that they have it as soon as possible, instead of a year later when they'll have to pay.
MS making mobile users pay for new versions of Windows 10 would blow up in their face really bad! LOL! I don't think they are that dumb