Even the 27" iMac has a 128 GB RAM SKU, so they're going to need to build a chip supporting external RAM regardless. External PCIe support already exists thanks

Even the 27" iMac has a 128 GB RAM SKU, so they're going to need to build a chip supporting external RAM regardless. External PCIe support already exists thanks to thunderbolt. So the architecture will be there, it'll just be a question of how expensive it is to scale up.