Here’s the point I feel like you’re avoiding grappling with:
Difficulty levels aren’t necessarily there to be well-tuned gameplay options for the game’s main audience. Especially these days, they are often there for those on the fringe of the audience: Those who aren’t comfortable with a controller in their hands, or those who don’t like the stress of severe challenge, those who want to spend their time absorbing the story more than butting up against the gameplay systems, or those who have physical disabilities.
I get that you’re not opposed to difficulty levels, presumably for any reason, but what you are saying is that if a designer adds an easier mode for these kinds of marginal groups, that they’re obligated to add gameplay incentives to the standard difficulty levels.
You’re thinking about difficulty levels in one way–with respect only to the integrity of the internal gameplay systems–but they serve a lot more than that. So while I don’t think it harms the experience of a disabled player for the game designer to give achievements to more typical players playing at the typical difficult setting… I think you’re wrong to insist it’s an obligation.