You are somewhat confused about data management and rendering. The PS5 I/O system doesn't help in any way to improve rendering (which is entirely done by the GPU). What it allows is fast movement of the highest detail assets possible. Look, it's simple to understand: just imagine the best possible graphic quality the PS5 could render, in most detailed, closed space, on rail game possible (so, all the GPU resources used to render the maximum level assets with the most possible quality). Now, in the past generations, the weight of the most detailed assets would be too high to be possibly used in an open world, because they fill RAM with only few detailed assets and the streaming capabilities of the previous generation cannot load those assets quickly enough to move in an open world at a reasonable speed.
In this generation, the I/O didn't simply grow proportionally to GPU power, but ten times more. There is now the possibility to stream the most detailed assets and geometries on the fly (even Z-brush ones, if one wants), allowing to have the maximum graphical quality of closed, on rail games, on open worlds also. That in theory, though, because an antire open world with super detailed assets would have a final dimension weighting on the terabytes, so trade offs will always be necessary. The point is, that data management speed in no more a limiting factor on the level of detail. And that it's only one possibility: you can link seamelessly different kind of gameplays (for example, maximum detailed flught combat, going seamelessly to ground, on feet combat, or looking from above on an entire 3d map like Skyrim one, and diving directly on a spot being able to play from the sky to the ground; or freeing game design from elevators, door bumpings, or other trick necessary to hide the loading of the next area (which also forces gameplay to be limited to the area present in Ram - a thing that made me furious, when I discovered that GOW Ragnarock will come out on ps4 also).
Rendering detail is no more the limit, now (as some wonderful games and the Matrix demo already showed). This gen limit is the mass storage memory. But if the developers want, they can create games with virtually any level of detail. Graphical level can reach nearly photorealism anyway.