It's not the same panel (different pixel structure for a start) and there are differences to be seen, I have a C8 here and use it when not reviewing TVs. But, I get what you're saying in that most end users wouldn't notice the improvements.Most people would not see any difference in picture quality between the CX and the 2018 C8. It’s the same panel with just some software tweaks, especially if you are not gaming.
OLED has not advanced much in the last two years. C8 was a good step up from the C7, but since then improvements have been marginal at best, except for gaming... not worth upgrading yet in my opinion.