They are offering this only for this year's Iphones and at some point this year's Macs. It doesn't seem like you can get a battery for 3-5 year laptops nor that you'd be still be able to get a battery for this year's laptop 3-5 years from now.
That probably remains to be seen. I imagine batteries are one of the most desired parts that they're going to make available, and assuming that they put a healthy Apple-sized margin on the prices, why stop selling them after a new product comes out?
It's possible but the reasons not to would be because the margins on a whole new device are larger and because carrying and producing old parts has costs.
Don't they still offer battery replacement services for those devices? If so they're already bearing the cost of producing/carrying these old parts. In fact, they could potentially make more profit selling these parts by themselves to end-users considering how much demand there is for things like batteries.
