Another thing people don't realize when they say that adding an easy mode doesn't hurt the game is that development time is a resource.
From Software dedicated a lot of time and effort to refine the feel of the game, so adding a different difficulty level isn't as simple as changing some values to make enemies hit softer or your character hit harder. Some people are happy with this and maybe more items or a rewind mechanism, but that probably wouldn't be the answer for the devs.
To keep they quality level up, the team would have to rebalance the game and test everything. Speaking as a a software developer, even though a lot of code is reused, the rebalancing and retesting isn't trivial.
Time invested in a difficulty the devs likely don't want in their game unless it provides the same type of quality the base game does means less time spent on the main game. Or if they add it after like some people think they should do, it means less time on the next game. Development doesn't just magically happen and developers have to put in time and get paid for their efforts. So it actually can inevitably affect the main game for the original fanbase.
Games done by Rockstar and Nintendo can include it all, because they don't have to limit budget time. Their games typically have a broader appeal and sell in the 10s of millions of copies.
But action games don't sell like that. Even DMC 5, a lauded game by both critics and fans, will be lucky to hit 5 million units over its lifetime. And even though it has the option of difficulty built in, the base game takes a third to half the time to beat as one would beat Sekiro or Bloodborne. Probably less for Dark Souls.
When it comes to options, graphics, whatever is in a game, if it exists someone has to build it. That means time put into that takes time away from something else unless a studio has a blank check (which most don't since smaller studios live and die by the next game).