A surah is a recognizable literary and recitational unit. It may incorporate several themes, but its beginning, ending, and mode of address distinguish it from other surahs.
One may create a thematic compilation of legal verses, place exhortations elsewhere, and collect narratives separately. That exercise can be useful for study, but it is not identical with the original arrangement in the muṣḥaf. The interpretive relationships found in a surah’s actual discourse must not be lost.
For example, when a narrative about a community leads to moral admonition, the important question is not merely which two academic disciplines are represented. It is how the narrative serves the admonition to the audience addressed.