There are several Japanese composers who are taking the melodica very seriously as an instrument to be composed for as well as to be used in composition. One example is Makoto Nomura, but he’s just the one I can find right away. I think Suzuki’s interest in producing higher end melodions is also indicative of an interest in going beyond its widespread use in music education in Japan.
Interest in the melodica by classical musicians is another manifestation of its being taken seriously, and it seems to be Japanese classical musicians in particular who are taking interest. If Kyoto (who recently joined this site) is monitoring this conversation, anything she has to say about this would be of great interest, as she is a professional classical pianist. Serious musicians who started out early on the melodica (as would be common in Japan nowadays) are probably apt to be more open to its possibilities, as with Jon Batiste. (And also players/composers like Hermeto Pascoal who aren’t hung up on what is and what isn’t a musical instrument.)