RJs are a rarity in Japan will continue to be so because of the economic setup in Japan & slot situation at the big two domestic airports.
80-100 seat RJs can be viable on many of the runs from Tokyo/Osaka to some of the smaller regional communities that only see one or two A320/737-type flights a day, but only if the number of slots at HND
doubles. Otherwise, any Japanese carrier would only need a handful (less than 5) for connecting any two small cities in Japan. Is another type of aircraft useful in such small numbers; most likely not.