I think consoles will have to be able to cache games from external HDD or SDD anyway
I don't see why this has to be true in any way. I think simply demanding games to be installed, and installed means "on the internal NVMe drive", would work just fine.
QLC only, especially in a none user-replaceable drive
What I'm predicting is explicitly just a NVMe flash drive, which can easily be user-replaceable.
At least throw in 64GB of SLC or MLC for that!
The total amount of written storage per dollar you get is more if you buy QLC than if you buy a small amount of SLC or MLC. You just also get more capacity, so it's less in terms of full drive writes. It's really hard to spend all the writes on a drive unless you stream a lot of video on it, or use it as a cache. The few megabytes of save data really *don't* pile up, not even if you save every second, and even the "daily suspend of 32GB" is just 0.000007% of the write budget of a 1TB 500-full-writes drive. Assuming even halfway decent write leveling, the only way it is actually possible to spend the full write budget of the drive is to use it as a cache of some sort, or stream video on it.
even if that means sacrificing support for industry standard user replaceable HDs
NVMe is the industry standard interface, and by late 2020 most new consumer drives sold will likely use it. Right now it's still in ramp-up, but uptake is rising rapidly. If they want to, they could easily fit two NVMe slots in the machine, to allow not just user-replaceable but also user-expandable storage, without having to toss the existing drive.
What I'm proposing is not anything as radical as Sony going "you have to use our proprietary, non-replaceable storage". It's just them saying "NVMe is the new industry standard, and it gives much better performance. You now have to use it for games. Also, you should use a PCIe-4 one for best performance".
Beyond just using NVMe, I suspect that they have a bit of "special sauce" designed to make the flow of streaming data from the drive to ram be as smooth as possible, maybe even with hardware units that uncompress/unpack some common formats or something. Another possibility that helps is just using a huge blocksize in the drives. This isn't really a detriment for almost any gaming loads, but will reduce the overhead of transferring data.