A treatment for level zero, is easily a separate book, maybe a setting and-or adventure.Sure. I thought it was apparent that I was talking about the three different books angle.
A treatment for epic tier, is easily a separate book, and if characterized as a superhero genre, can be setting or campaign arc from levels 13 and up being the separate book.
I get that WotC needs to know a project will earn profit.The front end costs of putting together a book like that are large. They then have to sell a lot of copies to break even, and then after that mass production costs are low, so they begin to make money.
It's worth it to make a level 1-20, 0-20, 0-24 or whatever single book, because they will sell a lot of those.
Meanwhile, each tier feels so different from the other tiers. It is worth it to handle each tier in a more specialized treatment.
Think of all the players who want a "low magic" setting. They know full well that they can ban fullcasters and stop at level 8. But they want a special treatment for this anyway.
Maybe these low magic fans can handle the idea of the tier of levels 9-12 (equivalent to the oldschool "name levels") is when characters build strongholds and the feel of game shifts as a nice cap.
A setting/arc book focusing on levels 1 to 13, can detail how the apprentice tier (basic 1-4) feels different from the professional tier (expert 5-8), and different again from leadership tier (champion 9-13).
A setting/arc book focusing on levels 14 to 24 for superheroes, can detail how each tier shifts in feel from mortal heroes to superhuman heroes to immortal heroes.