I think it’d be a bit more difficult than you are assuming however since it is also a multiplayer game. I guess they could limit the difficulty options to offline players only.
For online play, you can just match people who are using the same difficulty, and perhaps disable PVP entirely for Easy difficulty. For everything but PVP, it's not that big a deal. Seamless Co-Op Mod works with the Easy Mode mod. If modders can make a coherent experience, I'm sure From could. If they wanted to. Which they don't.
It's not about the reflexes, though. It's that they have opted to take actions that make the game unenjoyable for a significant number of gamers despite it having no positive benefit for ANY gamer.
And much of my Letsplay complaints weren't about Elden Ring. I mod that. I don't have the tools to mod Bloodborne, a game I otherwise would like more than Elden Ring because of its storyline.