A (the?) chief reason to use unified RAM was simplicity for development. If Sony knew MS were going with scratchpad RAM and devs would still have to design for it, perhaps they could have said, "sod it. Poor devs are stuck with annoying scratchpad RAM. Let's just go for performance." Then they could have put in 128 MBs of 400+ GB/s eDRAM coupled with 4 GBs GDDR5 or 8GBs DDR3 and provided a very different experience, whatever would be needed to make a better overall device to the current PS4. It'd be interesting to see a next-gen console with zero bandwidth limitations like PS2.ESRAM is pretty wasteful from a transistor budget standpoint. I'm sure Devs would have been a lot happier with 192-256MB of EDRAM. Perhaps that would have posed problems in terms of what process and foundry you could use. Challenges like that may have contributed to Sony's decision to go with unified GDDR5 even though they also explored an embedded memory design.
But then the SOC would be very different. Either larger and more expensive, or with less CUs. Still, HSA with really fast eDRAM seems quite compelling to me.