If Spiderman on PS4 was the first game and Miles Morales was just a stop-over spin-off to tide us over for the "real" sequel, then I don't think it's unreasonable to expect a major visual leap, especially when the sequel is on a console much more powerful than the PS4.
Knowing the visual jump between TLOU1 and TLOU2, it would be hard to get excited if TLOU3 on PS5 ended up being only a slight improvement over TLOU2. Same thing here.
If Spiderman Miles Morales had been PS5-only and a gigantic leap over PS4 Spiderman and then Spiderman 2 was building off Miles Morales then no problem, but that's not what we have here.
I'd actually argue the polar opposite to what you're saying, i.e. when did we suddenly stop expecting major leaps between consoles? People said the PS4 was terribly underpowered from the get-go and yet we still had visual bangers a few years in (relative to PS3). Hell, look at stuff like Ryse Son of Rome, what an absolute visual powerhouse and it was available at launch.
PS5 launched as a system with respectable performance (not top of the line PC-beating specs but consoles never are) and yet now it's somehow criminal to expect meaningful visual leaps?
I think what's happened is that, because of the relative of ease of putting games on both current and last gen (because they're both x86-based and both AMD, as opposed to PS4 x86 versus the Cell processor in the PS3), the executive teams (ie people controlling money and budgets) at these publishers/platforms have decided "fuck it, who cares about technical innovation, let's keep cross-gen going for as long as possible so we can milk the last gen installed base".
I get and hear what people say, and will always say... but all I can say.. is that they are wrong. Diminishing returns is a real thing. That's not to say we would never get another generational leap, but it's just going to take a lot longer to happen.
You seem like a reasonable poster, so I would give you the respect and courtesy of my time and respond to you properly.
To best make my point, here is the list of GPU power in some generations of consoles.
PS2 = 0.0062TF
PS3 = 0.23TF ( 37x more powerful than PS2 GPU)
PS4 = 1.8TF ( ~8x more powerful than PS3 GPU)
PS5 = 10.2TF ( ~5.5x more powerful than PS4 GPU)
See what the problem is? Those generational leaps people talk about, that you are referring to, can only happen when you are looking at jumps of at least 8x in GPU power. But that's even just scratching the surface, in reality, you should be looking at jumps in excess of 8x across the board. With EVERYTHING. RAM, bandwidth, CPU power, GPU power...etc. And I am sorry to break it to you, but that's just not happening from gen to gen anymore.
We basically need to get to the PS6, before we see a generational leap from the PS4 akin to what we saw from the PS3 to the PS4. And that's hoping that the devs don't somehow blow all that power on 8k or something stupid like that. Because thats another thing, these jumps we are talking about get totally negated if the resolutions are jumping too. Eg. You need around 6x the GPU power, to take the exact same game with a myriad of modern features running at 1080p to 2160p at the same framerate