It's an incredibly difficult thing for Sony to market clearly. If the PS5 truly can improve visuals by streaming in higher quality assests than the XSX is able to, how are they going to show or prove it if the 3rd parties don't leverage it in multiplatform titles?
The Demon Souls developer saying they are streaming 4GB/s of compressed data or people bringing up the Unreal demo makes no difference to the skeptical because they can't verify one way or the other if there would be a quality hit on another platform and developers are known to bend the truth to pander.
Taking a developer at their word doesn't help a layperson who wants to see the difference and if the loading times of the same games are similar but one box has bigger and easier to market numbers like teraflops, then that's an easy tie breaker for a lot of people.
It's the job of the Sony or developers explain the benefits because it's not reasonable to expect every consumer to be technically minded.
Just look at how many people don't know the difference between software and hardware emulation and draw invalid conclusions from comparing BC titles.
My point is, Sony still hasn't proved that their box can do anything unique. I personally do think there's something to the PS5's SSD regardless of developer praise simply because l don't think Sony would develop a custom storage solution if it didn't benefit them in some way.
It would be nice of Sony put out some sort of Demo showing a difference. Something like a scene streaming in textures or geometry and showing how quality can be limited do to low bandwidth.
I understand your point and agree with what you said. But, as you just noted, it is a challenge. And though bigger GPUs are more easily marketable, Xbox has failed to demonstrate that advantage. They didn't have any first-party exclusive to show that advantage (like Demon's Souls), or a demo specifically created for it (like the UE5 demo), and also failed at third-party performance advantages (AC:V, COD, DIrt 5) so far at least.
Sony didn't make PS5 for on-paper specs or marketing material. DualSense is another thing that couldn't be marketed properly, but Sony invested in it anyway. This is truly a console for developers and will yield many tangible and many more intangible benefits for users. One of such big benefits is Sony's goal to reduce development time by 6 months.
Demo: The UE5 demo was that demo, and it showed the capabilities of the PS5 perfectly. Tim Sweeney heavily implied that a demo like this will scale down on other consoles. But I agree a comparison would be even better.
It's capabilities will also shine more in first-party games. Moreover, when UE5 eventually comes, that will really show the differences. UE5 Nanite is all about virtualizing geometry, and it depends on I/O throughput. A demanding multi-platform UE5 game will likely perform much better on PS5, and the disparity will be noticeable. Epic also mentioned that they rewrote UE5 to leverage the SSD and I/O tech that Sony unlocked.
By 2022-2023, these differences will become very prominent.