I don't think the important question is "how much effort to make a quest?" but "is there a worthwhile story to be told for this event?"

Unless they have an actual good story idea for the quest, then I would rather it be kept short and sweet. Too often they drag out seasonal quests with trite paint-by-numbers "emotional" stories where a simple one would do.

Also, on pure numbers for this specific Valentione 2023 event, it's not "just" the one quest because you also have the series of vignettes afterwards. They're still fairly short and it would be nice to get more of them, but it's still an example of the quest extending beyond the single quest.