Not necessarily. It's sort of like Umbreon and Espeon in RSE. It no longer was dependent upon night or day, but on time. And XD even introduced stones to evolve them. Almost certainly, there will be SOME way to either transfer Pokemon to the new games, or a new method WILL be devised. Would be pretty poor planning on their part to make ANY Pokemon limited to only one series of games.
I easily say their will be Stones that are part of the rocks/Mt. Coronet.
They might put the Moss and Ice Rocks in the 5th Gen Region somewhere.
I always figured Moss Rock and Ice Rock were just giant evolutionary stones. As for Mt. Coronet, remember, it's just a "special magnetic field". All they need is some sort of Power Plant or maybe a held item. Somewhere with similar properties, you know.
I imagine that Mt. Cornet causes evolution through a strange electromagnetic wave. Perhaps future gens will have other places that emmit this same wave, or a scientist will try to re-create it allowing Magnezone/Probopass to level up there.
The Moss rock most likely causes evolution through the strange moss it holds. Not that hard to imagine that moss growing elsewhere, and as for the Ice rock, someone may bring peices of ice from Sinnoh that contains the same power.