Yesterday Forbes published a pre-launch interview with Ion Hazzikostas who is the Game Director of World of Warcraft at Blizzard Entertainment where they talked about World of Warcraft Classic.
Ion emphasized again that layering is a temporary solution to ensure the long term health of the realms. From his statements you can tell that Blizzard does not expect a significant amount of the current players to stay long term which is why they are so opposed to using additional servers instead of layering.
Worth note is that they plan on reducing layers even while populations are high as players will be more spread out:
Once the populations spread out around the world and people aren’t all in those handfuls of starting zones, we’ll be able to have even half as many layers as we did initially, with the goal just a few weeks into launch, as we go to phase two, of having all layers stable and having a single layer for each realm as the end state.