Elden Ring sold more than 20 million copies, that's quite a big "niche" if you ask me.
Not sure how lowering the health pool or damage per hit of bosses (as a very trivial example on how to easily reduce difficulty) affects the story of this game. And even if this would make the game less authentic to some players, they could just play it at the default difficulty..?
There is just absolutely no reason (other than maybe ego problems, but just add an achievement for each difficulty level then) why more difficulty options make the game worse for players who enjoy the current difficulty setting, as they can simply stick to the default difficulty. These players will have the exact same experience as they have now, and others who struggled or just didn't enjoy the grind of the default difficulty could turn it down a notch and enjoy the game.
They don't owe you anything in a sense that you don't have to purchase their product, that is correct.
Also yeah, the idea of cheating didn't even come to my mind. We used to do that a lot back in the day :D - but to be fair, trainers aside, games often actively supported cheats out-of-the-box, and I don't think From Software's games do. It's probably still trivial to cheat on the PC version, but on console, it might not be feasible.
I totally get the feeling of accomplishment that comes with playing games on high difficulties, I do play quite a few games at higher difficulties, but then again I also enjoy lower levels of challenge at times.
It's still a very valid complaint that difficulty levels aren't a thing. It wouldn't change the difficulty for anyone who enjoys the current default difficulty, and might make the game more enjoyable to other players.