I don’t think it’s a huge deal that Debussy didn’t write symphonies or concertos, because he did write 4 ballets, numerous orchestral works, numerous compositions for voice and orchestra (such as “Printemps”), for instrument (such as clarinet, piano, harp) and orchestra, numerous solo piano works, chamber music (such as “Danses sacrée et profane”), piano works for 4 hands or 2 pianos, works for voice and piano, and works for a cappella voice. Maybe he just didn’t like the symphonic and concerto forms and prefered more non-structured forms.
It seemed as though one was not considered a major composer until he had created a symphony, a concerto or an opera. Although D did compose an opera, I believe.