However, you have to keep in mind that duplicate assets will be a thing of the past, there will be much better compression in hardware, and you will be able to install parts of a game. It'll depend on how ambitious the game is, but overall they might actually be smaller than we're seeing now. 

Didn't Unreal 4 already have dynamic global illumination? I remember it was just too expensive for current gen consoles, so they ended up replacing it with a number of graphical effects that were less demanding. We will see, but I got a feeling that path tracing, like we're seeing in Minecraft, is a bridge too far for next gen consoles, at least when we're talking graphically demanding games. I hope I'm wrong, though, and maybe we will see some smaller scale games using it, or games running in 1080p. I mean Series X should be better at handling Ray Tracing than the ps5, but they also had to go with 1080p just to run Minecraft. My guess is that for path tracing we're going to need at least a RTX3*** gpu. However, Dynamic Global Illumination like Lumen looks like a fantastic alternative for next gen consoles, as its far less resource heavy than full blown path tracing.  

To me the SSD enabled tech does sound like it will the biggest break-through next gen when we're talking about consoles. Me thinks we'll see a much bigger leap than going from ps3 to ps4, because of it. But I also think it will be a much bigger limiting factor in cross-gen titles that still have to run on a HDD compared to just having a weaker gpu. Scaling things on the gpu side is one thing, but scaling the entire world design is going to be a different matter entirely.     

So where is the rest of the game when you only have installed a part of the game, when you reach the next level or load a savegame in a totally different part of the game?

Will the suddenly needed part then be installed from the optical disk with 0.02 - 0.05 GB/s (20 - 50 MB/s and very bad seeking speeds on the disc) or from the internet servers with 0.001 - 0.1 GB/s (1 - 100 MB/s)?

And how much is "much better compression"? 2x or 3x of current compression techniques? More? Less?

I'm also getting mixed signals about the advantages of the 5.5 GB/s PS5 SSDs compared to the 2.4 GB/s XBO SSDs or even slower NVME or SATA SSDs... will the needed data transfers per area of the games be huge (so the faster SSD is in advantage by speed but in disadvantage by capacity) or won't the needed data transfers grow compared to current gen systems due to better compression (so slower SSDs are still "good enough" to load the assets fast)?

You can't have it both ways.

I think they are more talking about things like only installing the multi player part of a game to save up space.

We will see how both consoles compare as they're actually quite a bit different in design philosophy, with both having its pros and cons. Series X has the better specs from a traditional standpoint, while ps5 excels in something that hasn't been much of a focus for developers in the past. It would've been way easier for Sony to just slap in a 1TB 2.4GB/s SSD, but they believe going to 5.5GB/s will make a big difference, and its why they're calling it the key to the next generation.

My guess is that we're not going to see much of a difference in multiplatform games, but exclusives should be interesting, though.