No one stated it won't get better, there certainly will be a leap. However it won't be as big. The proof is in the previous gens. PS1 to PS2 was a way bigger leap than PS2 to PS3. PS2 to PS3 was a way bigger leap than PS3 to PS4. However it wasn't as big as PS1 to PS2.
There's really no way to quantify the jumps. There's no delicate tool to objectively measure fidelity.
The best we can do is just try to honestly compare how amazed we were the first time we saw new gen graphics.
Hand on heart, when they showed that walkthrough of Nefarious City on the Ratchet State of Play, I was as genuinely impressed as I can remember being for anything from the PS4 generation.
The best the previous generation did was PGR3, which looked fantastic at the time, but it wasn't on some other plane of impressiveness.
The PS2 generation, there actually wasn't that much at the start tbh. Something like Ridge Racer V wasn't that amazing and definitely less so than PGR3 or Driveclub. Yes MGS2 was mindblowing but it came out a year later.