What exactly have 3D-platformers on PS4/X1 better than Mario Galaxy besides better graphics?
Bigger more interactive worlds. Dreams is not possible on the Wii. Dreams (like games) will vastly benefit from SSD since levels won't be limited anymore by what fits in memory.
What exactly have FPS games on PS4/X1 better than HL2 besides better graphics?
Bigger more interactive worlds. Tlou2 has vastly bigger environments than HL2 which you can even get lost in. So much more detail to hide in and hide traps / enemies is. You can't hide and crawl through tall grass in HL2 for example. Lighting plays a huge part in visibility and remaining hidden. SSD will benefit these games as well, no more limit of interactive environments since every change can quickly be written to SSD to be retrieved instantly when circling back. More complex environments provide more complex stealth game play.
What exactly have puzzle games on PS4/X1 better than Portal 2 besides better graphics?
Games like Infinifactory are not possible at that scale on PS3. This already has trouble chugging along on ps4 pro
SSD will vastly benefit the build/create puzzle game genre.
What exactly have Kart-Racing games on PS4/X1 better than MK8?
Track builder / VR (Track mania Turbo) Also here SSD creates opportunities to have everything on track interactive and no limits to track builders. Anything you knock over, leave on the track can be saved and retrieved from SSD in the next lap without worrying about memory constraints.
What exactly have open-world games on PS4/X1 better than BOTW besides better graphics?
Again bigger more interactive worlds. Fallout 4 base building, is that possible on Switch?
It was still severely limited by ps4 RAM and speed. (That above there only managed 6fps on base ps4 before the pro patch) The draw distance also left a lot to be desired. With an engine like the new Unreal engine with the new SSDs to no have constraints on how much you can build or it effecting performance would be a game changer. True interactive world where anything you touch stays the way you left it, no matter if you come back in a minute or a week later. No more memory constraints to have every (local) change have to fit in working memory.
So NO, the only thing that these consoles were holding back was the graphics department. PS5 games are not gonna be magically better than PS4/X1/Switch games just bcuz developers have more power to put more details into them, or should we expect Sackboy to be the most complex 3D platformer in history just bcuz is being developed for the PS5?
And I said it before and I will say it again: the only game that Sony has shown so far that possibly would not be able to run on PS4 is Ratchet & Clank (and even that is in the air bcuz we have seeing portal-jumping mechanics since.. well, Portal) everything besides that is just graphics.
Oh! and I'm actually expecting PS5 games to pop up more (visually) at first glance than XSX games, but that is because MS has committed to 60FPS and Sony hasn't.
As for your question: you should know by now that the hardcore crowd (A.K.A. the people who buy the most games) tend to upgrade on the first 2-3 years of the console cycle, so after that it makes little sense to keep supporting the old consoles since the volume of software sales just not make it worth it anymore. So yeah, both MS and Sony's decisions in this subject are purely business-driven.
So, yes. We are on the verge of a paradigm shift where working RAM is no longer a hard limit and can be augmented by the enormous increase in I/O that the switch from HDD to SSD offers. The ability to have 200x to 800x faster read/write random access opens up tons of new game play opportunities. Stuff we haven't seen on PC before since that also still has to support mechanical HDDs. (But PC does have the benefit of a much larger working RAM pool)
We can finally leave the static world design behind. Evolving worlds, making real changes in RPGs and god games. Game design will change to make use of and compensate for evolving worlds. Weather can actually change the worlds. Earthquakes, bombs that flatten entire areas, burn down forests, alter water ways. The only downside, save games will be massive in size :/
And Minecraft might finally run decently on consoles.
Bigger worlds with more details in them is what we always expect by default in a new generation, and those are things that can easily be scaled up or down. Even open-world games can be scaled down to run on much weaker hardware, even if you have to cut the world into several chunks, limit the draw distance, and put loading screens on the weaker version; I mean, we have already seen it multiple times.
And I'm not sure what you mean by "more interactive worlds," interactivity comes down to game design, not hardware power; I mean, HZD was designed with PS4 in mind, yet BOTW world feels more interactive. And we have been seeing destructible environments since Battlefield Bad Company 2, fire burning down forests since Far Cry 2, weather changing the environments in FH4, etc.
And I don't know what you mean with TLO2 hiding mechanics. We have had hiding mechanics for ages, the only difference here is that the grass looks more realistic.
Look, I will concede that when the gap is too big there might be some things that you might not be able to do on the weaker hardware, but I do not believe that the gap between 8th and 9th gen is so big for that to happen, so I will believe this "new experiences not possible before" talk when I actually see it (and I'm still not convinced with this portal-jumping mechanics in R&C bcuz he have seen portal-jumping before, ages ago actually).