Forget about accretion disks, that's a local phenomenon.You're right in the sense that galaxies exist because the Black hole at the center does not become massive enough to have all the matter. In the same sense that earth exists because of the sun because it the sun is not massive enough to have subsumed the earth. Also, nebulae are gas features within galaxies, but entire galaxies we're once called nebula before we realized they're just far away galaxies.