There are no well-reasoned arguments for why adding a difficulty setting would compromise core experiences of the game, because the core experience of the game would be untouched. If the player "can't resist", that's their problem, not the developer's. This is, again, like arguing against the existence of bumpers in bowling lanes because you don't think you'd be able to resist using them.
Sekiro is a game carried entirely by the experience of just barely slipping by. It's about being forced to focus and pay attention in a way that the vast majority of games do not expect you to do. What makes the game work is that the game is carefully tuned in a way that allows the player to gradually learn it as they progress. Adding more difficulties or options to give players the ability to fine-tune the gameplay has a nontrivial effect on how much work it takes the developer to curate the difficulty. If you add an easy mode, what you're left with is just a shitty action game that retains very little of what makes it good. To avoid that, the developer would essentially have to duplicate the work of curating the "normal" mode except now they need to imagine a player whose skill level they can't even really define. And then after all that if the game still turns out to be too hard for some people - and it will, because being challenged to pay attention is
literally the point of the game - you'd still have failed to address the criticism that some people simply can't beat Sekiro.
If your argument is that developers should include options that allow people to make their game bad for the sake of letting people beat it, then you'd do best to just say so.
I also reject the argument that Sekiro creates some kind of a reaction time barrier that some players are physically incapable of overcoming. While this is probably true to an extent, the fact is that a lot of people have beaten Sekiro and they're hardly genetic freaks. Indeed, I have a rather poor reaction time and was able to beat the game without too much trouble. Some players may have gotten through it faster than I did, but since my pride isn't wrapped up in being able to beat videogames that's not a huge deal to me.
Finally, even if I'm wrong and Sekiro would be a good game even if it was easy, there are entire genres of games for which difficulty and a massive barrier of entry are defining traits. I am a big fan of the Tetris the Grandmaster series. The third game in that series is so difficult that it might as well be impossible; the number of people who have beaten it is in the single digits despite there being a decent size community of players who have been trying for the past fourteen years. Indeed, it's probably true that most of the people who play the game are fundamentally, physically incapable of playing Tetris at the speed required to play the game to completion. TGM's intrigue (not to mention fanbase) would disappear overnight if the games were easier to beat. TGM3's legacy is entirely tied to the absurdity of the requirements and the level of accomplishment it takes to overcome them. Why is there no concern about the accessibility of this game? Is it a sales thing? Could it be that the
real problem is that people don't like the idea of a AAA they can't beat?