So, a question then:
Do you have total control over
your own thoughts and actions? Do you always succeed at the things you want to succeed at and fail at the things you want to fail at? Do you only feel fear when you wish to, and never when you do not wish to?
Because by the standard you're articulating here, most human beings rarely if ever have agency. Anyone with a phobia, for example, simply does not and cannot have agency, because they don't get a choice, they feel the fear and it can be crippling. I, personally, have phobias of spiders and of falling; I don't mind being
in high places, even ones where I can see how high up I am, but I am
deathly afraid of falling, and will outright panic if I suddenly realize that the protections against falling are inadequate to actually
prevent a fall. Does this mean I as a human being can't actually express agency because I don't have control over this? What about (say) someone with clinical depression, or ADHD, or (etc., etc.)?
And this isn't even getting into, as previously noted, things in D&D which defy this description and have for ages. Dragons of sufficient age have a nonmagical, entirely natural (in 3.x, "Extraordinary", which is
explicitly not supernatural) "Frightful Presence" which (a) induces a roll (a Will saving throws), and (b)
forces the character to be frightened if that roll fails. In other words, for more than 25 years, D&D has had
exactly the thing you claim destroys agency, a forced roll which induces a mental state. And as I have said before, 5e continues this tradition, as the
completely mundane Spinosaurus Dinosaur has Frightful Presence, and the the equally mundane Battle Master being able to use Goading Attack or Menacing Attack, and nothing prevents an NPC from being constructed so (in fact, many fans have poo-poohed the separation of NPC abilities from PC ones!) Or if using the Battle Master as an example doesn't work for you, the "Warlord" creature (which can be "any humanoid") from VGM and MPMM has a Legendary Action that forces a DC 16 Wis save or else the target is frightened until the end of the Warlord's next turn.
So, what exactly is
different here? D&D still allows entirely mundane creatures or effects to strike fear into the hearts of PCs, if they fail a roll to resist it. What makes this different from the things
@pemerton has described? Why is what D&D does, and has done for
two and a half decades, acceptable, while BW is unacceptable? Per your own descriptions, D&D takes away your agency just as much as BW does!