Without massively increasing detail... all you've done in the scenario you describe is eliminate load times... and remove the need for RAM.... but.. we've likely doubled usable RAM this gen.. why again?
And what bizarro world do you guys live in where you've missed that the PS5 will be capable of massively increasing DETAIL because the PS5 can stream gigabytes of assets constantly?
"That's not the point"? WHAT?
Speaking in general, if last gen you had 1080p textures at most, now you'll move to 4K textures at most. That's the upper limit, you don't need more if you're never going to render at more than 4K. Textures are a decent chunk of the game file, so that is effectively "capped" no matter what. Also, if games use a system like the one shown today they can get rid of all the alternative LOD versions of textures, since the highest quality one is always used and scaled dynamically by the engine.
Models are not as heavy in comparison, and as far as I know they are easier to compress significantly compared to textures (them being vectors rather than rasters probably helps in that?). Logic and general code might increase in size for games that implement more complex systems, but definitely not exponentially. Sound should not be drastically different either, since all the additional technology for 3D distribution happens during processing and nothing suggests assets will need to be different to begin with.
There is also a huge re-use of textures and assets in games, because no developer could go and model and texture every brick and rock individually. This has always been the case, and won't change now.
On top of everything else, all this stuff is compressed on the SSD and gets de-compressed on the fly during the transfer to RAM (not for nothing PS5 has a dedicated decompressor chip). Going by Sony's data of transfer rate for compressed and uncompressed assets, it's probably a further reduction by about a factor of two.
Regarding your reasoning, being able to transfer 8-9 GB of effective assets a second doesn't mean you are going to move hundreds of thousands of GB of different assets form the SSD in a matter of minutes. First of all because you will be streaming the same stuff in-and-out constantly in similar environments. Second, because most transfers need to happen at a frame or sub-frame pace, so they couldn't occupy GBs of their own to begin with, not even at these speeds. If you assume you are in an environment and you see 33% of it at any given time, that 33% can't weight 12-15 GB on RAM (let alone on disk) for the simple reason that you could not possibly conjure up the remaining 66% fast enough (rotating completely in one second, which is relatively slow, would require you to pull out an additional 25+ GB of stuff, which you obviously can't do in a second).
Game size will probably grow in the end, but once we take into account an approximate 20-25% reduction due to no longer needing to repeat assets in multiple points of the storage, I'd wager for most games we're looking at something in the range of 75-100GB rather than the 40-60 GB we have now. Definitely nowhere on the same scale as the increase in disk transfer speed (x 100).