The Fusion drive is just a bridge product until large SSDs are "affordable," meaning no more expensive than a HDD. In three years time the only moving parts in the iMac will be the fan and the speaker cones.
So the iMac of 2016 (thinking of when AppleCare runs out on the new models) will have SSDs, a base 16GB of RAM, the new Wifi technology (I forget the nomenclature but it is faster than current), and of course the CPU and display processors current in 2016.
Meanwhile I'm still happy with my 3 year old iMac, but probably won't be using it in 2016.