The use case people have described for alignment is that of a NPC who is not sufficiently important for the DM to flesh out his background beyond a shorthand.
Hence Arlas, the type of character a DM might create on the spur of the moment. It seems to me, that for this type of typical character, which unless I’m misunderstanding is the use case for alignment, alignment isn’t particularly useful: both because many alignments would lead the character they exact same way, and also because people of the same alignment (as modified by the notes the GM might have on a typical shopkeeper) might react differently.
What you have responded is that if the PCs have a completely different interaction with Arlas in the future, alignment might be relevant in that case.
This isn’t a particularly strong case for alignment for minor NPCs.
Arlas is better served by the 3 sentences of notes that I used to describe him (and that took 1 minute to jot down) and his alignment was irrelevant to describing him, or even characterizing his description.