Xilinx has dropped some hints on this, but I think the bigger issue, is the FPGA is FAR LESS price sensitive, and MORE performance sensitive, than the products that use Stacked-Die.
So, the top end FPGA's simply add more on chip RAM, and sell you a bigger part!!
You are quite right tho, that there is an opening for a smaller FPGA, or larger CPLD, tightly coupled to RAM.
Even here, the issue is volume, as the really big players like Broadcom, can get good enough performance from their Stacked approach, to tackle 98% of the high volume markets - that leaves the crumbs for the PLD vendor.