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.