The problem is when those "small diminishing returns" become inperceptible to the human eye. I'd say that is certainly the case for the 3090Ti versus a decent OEM 3080 Ti.
Having just looked at OCUK 3080Ti prices versus 3090Ti prices, the average price delta seems to be about £700. That's a lot to pay for an inperceptible improvement.
And for those cases where very low framerates come into play (Cyberpunk, AC:V, WD:L and MSFS2020 @4k/Ultra, for example), you'd still end up tweaking the settings anyway to get
vastly improved performance on
any card, so even those cases make little sense, IMHO.