I do think you are overthinking it a little and worrying too much about the Souls experience vs what some people who could potentially derive some form of enjoyment from the series that you and I might not be able to grok actually want. Dodge iframes, parry windows, poise, stamina, stagger, status effects, all of these are mathematical variables inside the game that can be easily controlled on the fly with global difficulty modifiers. More iframes to dodges makes one able to avoid attacks with less precision on their read and timing. Longer recovery times between enemy patterns can allow for greedy hits that would otherwise get punished which also means more damage on the enemy, less time it is alive and less time for you to make mistakes. Less stamina drain from attacks combined with more forgiving regen can make it easier to simply stand with your guard up and endure. Less incoming damage and more poise means that you can make more favorable trades of blows. Flasks with more shots and more healing mean that you can make more mistakes or trades between fires. All of these things add up to allow for more mistakes, less precision, less grind, less risk for more reward, less mastery. They can make the difference between dying 20+ times between bonfires and dying 3 times, which for some people takes the game from unplayable frustration to fun dark fantasy romp.

As you said yourself, you can use the systems to deal disgusting amounts of damage, or become disgustingly tanky and laugh in the face of your enemies, as a friend of mine did after getting real tired of some of Elden Ring's shit, but it requires mastery of the systems and the patience to achieve it. I know all the ways to make it easier. Some people don't want to deal with that noise. A difficulty modifier that allows them to mess with the numbers on the fly achieves the same effect in a way that is more in line with the amount of effort they are willing to invest in the game. And an "I really don't want to deal with this shit" godmode toggle covers the edge cases that maybe mathematics alone is not able to overcome. It's anathema to the game's design and intended way of engaging with it, yes, but literally just let people have fun in wrong and bad ways, in a singleplayer game it's not hurting anything other than their own experience. They may not want Souls but they might be on board with something Souls can made into via easy to implement difficulty options.

I don't think that any game should be designed from the ground-up with the notion that it can be played in any way you wish. If all weapons and skills are equally viable then there are no decisions to make and nothing ultimately matters. Power fantasies where you can do anything and be anyone and use whatever you wish are a bad design paradigm that leads to shallow and empty games. Absolutely agreed on this one. Ideally, games should have a clear idea of who they are aimed at and be built around delivering that experience rather than being an identity-less one size fits all product that is made to maximize mass appeal. Hell, most of the games I enjoy are niche products. I also completely agree with you that multiplayer games should be approached in good faith and that you should be expected to learn the basic systems of the game and hone them. If I am playing Helldivers and I am matched with someone who repeatedly shows that they are unwilling to engage with the game and respect its fundamentals, then I want to kick them from my squad. Especially if we are playing on the higher difficulties and they are complaining about game balance being supposedly bad rather than putting in the effort to learn the fundamentals and improve their gameplay before tackling harder difficulties.

I also don't think any developer should be expected to carefully tailor and curate every difficulty setting to the point that they are essentially making several different games either. That is indeed unreasonable. Keep it to easily tweaked variables and cheat code equivalents and if that isn't enough, then, yes, people should just find a different game to play. Make it clear what is the setting for the "proper" experience and be transparent about the scope and limitations of adjusting the difficulty. Ideally, every game should offer a demo that allows you to see what you are getting into and if the available scope of experience customization is enough to cover your needs.

What I believe is that how individuals choose to engage with singleplayer games is entirely up to them and I respect the fact that they might look for different types of experiences than I do. If these individuals can be provided with more choices to tweak their experience to their liking, that require low development effort and are put in after the game is designed around its intended gameplay loop and experience, then just do it and let them have their fun in peace however wrong it might be. Let people give themselves 99 flasks that heal them to full and and reduce the damage they take while increasing their own and boosting their stamina regen and even godmode themselves and play Ruined Medieval Dark Fantasy Walking Simulator instead of Elden Ring, idgaf, it's their money and their time. Hopefully they will one day think "maybe I should try the actual intended experience" and play the real Souls but if they don't want to that's also fine.

In conclusion my stance on difficulty in singleplayer games is:

• make games with a clear intention and intended difficulty and experience
• give the people a big difficulty tab in the settings menu where they can tweak various easy to expose parameters to their liking as well as godmode/infinite consumables/whatever buttons that do what cheat codes used to in old games. You do not have to completely redesign the game from the ground up for easy mode and no one should seriously expect this from any developer, just offer a bunch of tweaks that may be cumulatively effective in making the game more approachable to some people who are put off by the default difficulty
• clearly point out what are the intended parameters and what kind of experience the game is meant to convey while making it clear that tweaking the values will detract from how the game as art is meant to be engaged with
• let people tweak the values to have their incomprehensible terrible fun if they so desire, idk maybe someone really likes sightseeing around the maps in Elden Ring for whatever reason, I think I remember a twitter post by someone who just really liked the world and wanted to walk around and vibe so it might just work for some folks and if it's low hanging fruit development wise why not do it