I very strongly feel that it should be up to the designer and it completely depends on the kind of experience the designer wants the player to have. If the only reason a game has to add an easy mode is to prevent the players from getting frustrated, it might make the overall experience much more mediocre.
For a game like Dark Souls, a huge part of the experience is to fail multiple times until the player has mastered the skills needed to move on to the next challenge. Sure, the process of getting there itself might become frustrating by design, but all these failures will ultimately make the experience of finally overcoming the challenge worth it. For some games like this, getting frustrated is part of the process, and if these games adds an Easy Mode option just to ensure people don’t get frustrated, the experience of finally overcoming the challenge is very diluted. Adding an Easy Mode probably makes complete sense in games which are aimed at mass market, but for games which are made for a much more niche audience and the experience has to be just right, having an Easy Mode option will most likely dilute the experience. This is also true for other mediums as well, like films for example, where for the harder to understand films, I don’t want the directors to add a “here is what the film was actually about” kind of a note in the end just to ensure everyone gets it, since it will dilute the overall experience.
This is also exactly how I feel about majority of the criticisms towards games like The Witness.
A common analogy I used in this case is if I’m teaching someone chess for the first time, will I let them intentionally win every time just to make sure they don’t get frustrated? This will probably make them feel better in the short turn, but if I don’t and the person who is learning actually gets better, the experience of that person finally learning the skill and defeating me for reals for the first time will be much more profound than any of the tiny feel good moments that were not provided to them initially.
However, this is also a fine line between implementing this well and just badly designing / pacing a game and releasing it by saying “Well, Dark Souls is also hard, right? It’s obviously not the designer’s fault that players are quitting early”. Where the line is, is probably up to every individual player / designer.