The conceit of the point presented is a rather meaningless one. And specifically contingent to the 3e book itself. But of course it is your choice to freely ignore the point and argue otherwise.
If a so called "best" supplement already exists there is no reason for another product of that nature to exist if the quality cannot be matched.
As demonstrably evidenced quality is really subjective.
Agree with you to a point, but if that is the case, there would have been no point publishing CoS. (I have not read it, I only played the DDO adaption of it but I think it is eventually quite good)
Most the 2e red box Ravenloft and its supporting material, Van Richten guides, and the adventures each giving more depth to one domain, are unmatched by anything else D&D ever brought out in quality of designe, NPC richness etc. (not always in maps and such though) , so no need for 5e Ravenloft material according to your argumentation.