The Rising Shadow was abandoned because Tolkien literally couldn't make a story out of it. Youths turning to evil happens every once in a while, or society degrades, but I guess it happenning so close to the end of LOTR would be too odd, and there's no remaining major malignat forces left to either stoke a revival of Melkorism, Sauronism, or anything else. The Dragons are gone. Orcs are still around but vastly fewer than they ever have been and dying off. Elves are leaving. Something happens to the Dwarves, eventually, but we don't know what.
And in two ages, the continents take on their 'modern' forms, but at what year analogous to us we don't know (maybe the ice age period would be the 'fifth' age of some 200k years, and the 'sixth age' the final movement therafter?) and the men of the first ages are vastly different from the men later - remember, Tolkien was 'officially' translating a long-lost book, the Rohan aren't, say, Scythianized Goths running around in 500 AD, but something far, far older, before Europe even existed, Rohirric was just translated to Old English and Dwarven to Old Norse and Westron to English.
Rohirric sounds Dwarfish to me actually, with words like 'Kûd-dûkan', 'Turac', 'Logard' and the like, while Dwarfish sounds either more 'evil' or related to Semitic with words like khazâd, tarâg, shathûr, ishmêk; and of course this is all we know of Black Speech - Ash nazg durbatulûk, ash nazg gimbatul, ash nazg thrakatulûk agh burzum-ishi krimpatul, Uglúk u bagronk sha pushdug Saruman-glob búbhosh skai! -.
Ah, to be a conlanger.... working for six decades or more on dozens of languages....but I digress.
