Wanted to believe for the longest while that the type of solution you're talking about (3D-Xpoint PCM, ReRAM etc.) was possible, but I had the unfortunate fate of doing some hardcore research and I just don't know if they're going to happen.
I tried factoring out 3D-Xpoint (DRAM type)'s price per GB and figure it could get to about $1.50 with the bulk a company like Sony or MS could secure them. Even then, 64GB would be $96, and 128GB would be $192. And they'd still have to arrange it in a way to saturate the bus; IIRC a 128 GB module of Optane DC Persistent Memory gives about a bit over 8 GB/s of read performance and a bit less on write performance.
But at least 3D-Xpoint actually exists in mass scale with consumer and data market products; ReRAM's even worst, there hasn't been a single chip produced larger than a few MBs in size, for commercial production. So I think the timetable of seeing it employed in, say, PS5 at any significant capacity has passed. Maybe PS5 Pro (and/or XSX X), but not base PS5. They could probably spin 3D-Xpoint production into ReRAM production (they're both fundamentally the same technology at the root, but with some key differences in how stuff like writes are handled), but I personally wouldn't know if that's happened at all.
That's why when we've been talking about the SSDs in these systems, I've been picturing them like AMD's SSG line; 3D NAND custom-soldered to the PCB directly (or if they want to make them replaceable, on a daughtercard in a cartridge container connecting to a card edge interfaced through PCIe 4.0), given wide bandwidth and fast speed access to the CPU and GPU, and able to be memory-mapped. Probably 1TB in size, between 8 and 16 3D NAND chips in parallel (3Gbps I/O for 384 MB/s, x8 for ~ 3 GB/s, x16 for ~ 6 GB/s), that way they could go with either 1Tbit (128GB) or 512Gbit (64GB) 3D TLC NAND chips. And if they wanted, still connect more banks of NAND via serial connection to the main chips for more storage capacity (bandwidth and speed would stay the same though).
Sony and MS would want to keep that connection and the drive proprietary though, so they could have a second M.2 slot on the systems for user-optional secondary drives that act more like traditional SSDs. In fact I think the "odd" thin slot on the back of that leaked XSX picture might be an M.2 card slot.
... but i believe that, in the end (even if it doesn't happen), we're still getting a great overall jump with this new machines _ regarding CPU, GPU, SSD, RAM _ and everything else will be quickly forgotten! 
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