I feel that what's presented here is an intractable problem; there's no right answer, there are only aspects to the problem. I've thought about it a lot.

First, this is a legitimate example of a slippery slope. That alone is not a reason to avoid adjustable difficulty, but when done in the name of making a game more accessible it becomes difficult to pin down when a game's difficulty is the "correct" level of accessible. A wheelchair ramp is a wheelchair ramp; they work for practically anyone with a wheelchair, if built to a quantified code. But difficulty is particularly, well, difficult, to quantify. There will always be people who can't enjoy video games, because disability is a multivariate spectrum unique to every individual.

On one end of that spectrum, there are many people who despite being otherwise able-bodied and minded cannot beat Margit. A friend of mine was struggling with him quite a bit. A couple hours later he finally got it. He was ecstatic, of course, then said "I don't know how you beat him so quickly". I didn't. It took me about 80 deaths and three hours of aggregated attempts. That's the point; many people view Souls games as having some Elite Echelon of players who "get it", destroying every boss handedly, and while there certainly are players like this, the fanbase is predominated by people who experience exactly the same difficulty players who "give up early" do. What we're talking about, in these instances, is not an able-minded skill differential; we're talking about Patience and Willpower. I don't believe its fair to consider these aspects of human nature a variable in the "difficulty" argument; making From's games easier purely to cater to people who don't have the patience or willpower to push through fundamentally changes the experience they have; it would no longer be a "great" game, because what makes it great is predominated by its demand on your patience. Its weird; some people like that, some people don't. But you take it away and its unclear if what is left actually has a similar value; it turns out, maybe you just wouldn't like it either way.

Its similar to demanding that a scary movie have an option to remove the scary scenes, because I don't like to be scared; this demand would similarly be rooted in this idea that a tranche on the population is "missing out" on what many people love about scary movies, so they want to change the artform in order to enjoy it. Not all art is built for all people. Not everything should be a Disney movie.

Your argument that a difficulty slider provides options for players to find a level of difficulty which is "right" for them is cute, but it assumes that players know what this true difficulty should be. The problem is: we don't. How can I, able-bodied and minded or not, be trusted to make a decision on the "correct difficulty for me" of a game whose entire point is to be near-impossibly difficult? I die to Margit eighty times; should I turn the difficulty down? How about a hundred times? Two hundred? What is the right number of deaths that a player should experience facing Margit?

Some games firmly take the stance that the number is near-zero; that's absolutely fine, I love these games! From's games are not that.

Second: I've played a ton of games. Too many; many with difficulty sliders, many without. I've literally never seen a game implement a difficulty slider which resulted in a satisfying change in difficulty at the level of FromSoftware's game's difficulties. When games implement difficulty sliders, they almost always take the form of: start with a base level of health, damage, etc, call that Medium. Increase by 25% for Hard. Increase by 50% for Insanity. Decrease by 25% for Easy. Etc. It doesn't matter if its TLOU's Grounded, or Mass Effect's Insanity, or God of War's GMGOW or Fallen Order's Jedi Grand Master. Sliders ALWAYS, in 100% of cases that I've seen, mean: there's numbers within the matrix of the game's design, lets tweak those numbers up and down.

Someone gets shafted, in literally 100% of situations. Either a game was designed for medium, and the hardest difficulty is unsatisfying because enemies become damage tanks, or ammo becomes too scarce, or something. Or: a game was designed for the hardest difficulty, and the easier ones feel like an afterthought where enemies are tissue paper and you're a god amongst mortals. It turns out: game development is REALLY, REALLY hard. Its a miracle games are even released, let alone that a small number are great. Your assertion that a difficulty slider would help make a game relatively equally difficult among differently-abled people either would never work, as it hasn't in the past, or is tantamount to demanding game developers design and build multiple finely-tuned copies of their game for each difficulty. Not just health, ammo, damage values; but animation timings, ability sets, parry windows, item drop locations, its a herculean task!

Not to mention: your assertion assumes that the quadratically multivariate spectrum of human disability can be satisfyingly encoded into a linear slider. TLOU2 is the only game I've seen which has multivariate difficulty sliders; from enemy health to drop rate, you can tweak five or six variables. But: that circles back to my previous point; it becomes a slippery slope where players are overloaded with fine-grained control over difficulty in order to produce the perfectly tuned game for them, without any guidance on what that "perfect tuning" actually is or should be.