> The technical reason to put memory and the CPU/GPU on a single package is to decrease memory latency (and possibly boost operating frequency).

You've got that backwards, I think. Putting memory on package, or even just soldered on the PCB nearby rather than in removable DIMMs primarily helps hit higher frequencies or lower power at similar frequencies (GPUs and smartphones provide a wealth of examples). There's little or no impact on latency, because most DRAM latency occurs within the DRAM dies or inside the CPU's cache hierarchy rather than on the link between the CPU and DRAM.