Obsidian is free. With great respect for all the enthusiasm behind Obsidian, Devonthink is a lot more powerful. A lot of its power lies in what you can do with basic features. The potential tends to be revealed over time, not on a quick read of the user’s guide. Both are easy to use.
Arcs can probably be used any way you want to. I think the intended use is for plot threads. For instance, Luke Skywalker’s arc starts with frustrated farm boy. He becomes a curious doubter, then a despairing student, and finally a galactic hero.
You can create a category, called an arc, that would be the parent for every scene in that progression.
It sounds like that’s what you propose, so you’re right in line with the intended use.
It’s intriguing. I’ve used Aeon for years, always as a way to diagram a story already locked in marble, or the history of a situation. Using it as a potter’s wheel to evolve a story sounds fascinating.
I intend to give it a whirl.