@smugumin In BotW's case that's not how it is at all. The difficulty is largely artificial aka enemies deal more damage. Aside from that it's trivial stuff like "it's too cold, make a warmth potion". The graphic isn't talking about difficulty, it's talking about gameplay and narrative complexity, which only coincide with difficulty.
In BotW and TotK, every area is designed with the assumption that it's the first one you go to. The dungeon quests are all written according to the same template ("Link! The divine beast is messing up our home! You have to help us!"), and the dungeons themselves follow the same flow of "solve 4 disjointed and easily-cheesable puzzles to access the boss".
To make this design possible, the player is given all of the items they need at the start. So the set of possible gameplay scenarios is always constant. There's nothing unique gameplay-wise about the divine beasts beyond set dressing (and even that's limited). Compare this to old Zelda where you started off with only a sword/shield and every dungeon introduced a new ability which added a new dimension to gameplay. Similarly, since the next area you reach was usually thanks to the item you just obtained, the devs could write those areas as a continuation of an existing plot, whereas in BotW/TotK the "next area" is a deja-vu repeat of a story you already went through. That's why Link as a character never develops in these games. He can't. Worst part is you can fight Ganon at any time, so all those dungeons are just glorified sidequests. The overall plot does not progress in any meaningful way because even Hyrule Castle is written as though you skipped all the dungeons. In BotW/TotK's case, the graphic never even REACHES the yellow portion.
You can call it on-rails Reddit design, but that doesn't really make sense. An open-world game has the same amount of content, you just decide what order to do it in. However, that means the game will have a bland story and flat gameplay progression because the devs can't write a coherent introduction/rising action/climax story. For that you need locks and keys, which are not bad things in games. Case in point, the most iconic moment in BotW is obtaining the paraglider so you can jump off the plateau and explore Hyrule. Literally obtaining an item to unlock new areas.
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