pruwybenThere's probably a word I've been pronouncing wrong my whole life that I don't know about
tiredofsametabA "niche" is not a "nitch"
JamesBeanYou're a bit too late for trying to complain about that one.
The latter has been the dominant American pronunciation of the word for so long that it now appears as the primary pronunciation guide in American dictionaries.
Both Oxford English Dictionary and Merriam-Webster agree that “nitch” was the correct pronunciation in both British and American English until very recently. You already linked Merriam-Webster, so here’s O.E.D:
N.E.D. (1907) gives only the pronunciation (nitʃ) /nɪtʃ/ and the pronunciation /niːʃ/ is apparently not recorded before this date. H. Michaelis & D. Jones Phonetic Dict. Eng. Lang. (1913), and all editions of D. Jones Eng. Pronouncing Dict. up to and including the fourteenth edition (1977) give /nɪtʃ/ as the typical pronunciation and /niːʃ/ as an alternative pronunciation. The fifteenth edition (1991) gives /niːʃ/ in British English and /nɪtʃ/ in U.S. English.
(N.E.D is the original name of the O.E.D. “/nɪtʃ/” is pronounced “nitch” and /niːʃ/ is pronounced “neesh”.)