Britain might have declined slightly, but Marx's home country, Germany, has comparatively fallen from the top of the world, intellectually. It's still a heavy hitter, but if you compare it to pre-WW1/pre-WW2 Germany, it's just a ghost.
Why?
https://en.wikipedia.org/wiki/Gottfried_Wilhelm_Leibniz
https://en.wikipedia.org/wiki/Carl_Friedrich_Gauss
https://en.wikipedia.org/wiki/Georg_Cantor
https://en.wikipedia.org/wiki/Albert_Einstein
https://en.wikipedia.org/wiki/David_Hilbert
https://en.wikipedia.org/wiki/Heinrich_Hertz
https://en.wikipedia.org/wiki/Werner_Heisenberg
https://en.wikipedia.org/wiki/Fritz_Haber
https://en.wikipedia.org/wiki/Justus_von_Liebig
https://en.wikipedia.org/wiki/Wilhelm_Rontgen
https://en.wikipedia.org/wiki/Hans_Geiger
https://en.wikipedia.org/wiki/Max_Weber
https://en.wikipedia.org/wiki/Hans_Bethe
https://en.wikipedia.org/wiki/Bernhard_Riemann
https://en.wikipedia.org/wiki/Max_Born
https://en.wikipedia.org/wiki/Friedrich_List
etc, ad infinitum.