systems where the character is forced to evolve on an ongoing basis I'm not fine with; barring these occasional external magics I-as-player want control over how my character evolves and develops.
There's a tension here, I feel, between the last quoted sentence and the first two. Namely, that authoring a character's response or action isn't typically a manifestation of knowledge about the character. It seems more like a manifestation of hope or desire or preference in respect of the character.I have a preference for how/when/why the character is not in full and rational control to be up to the player of that character, rather than determined by a game mechanic.
From my standpoint the player knows more about the character than the game mechanics ever could
Not really, because I assume that a typical outcome of frightening someone by (eg) decapitating a prisoner in front of them and threatening them that they will be next would also be to inflict the frightened condition.Sure a DM could grant ad-hoc advantage on the saving throw (although I think ad-hoc adv/disadv is more common for ability checks than saving throws), but I don't see why the chance of success is pertinent? Even if there was a 95% chance to successfully save, when it does take effect the dragon fear is still both stronger than, and categorically different from, ordinary intimidation. A dragon can go up to a person it's never met and, without saying anything or knowing anything about them, have a chance (even if it's a small one) of inflicting terror strong enough to specifically stop someone from approaching (no matter how strong their motivation to do so) without actually incapacitating them or preventing them from doing anything else. Somewhere between 6 and 60 seconds later, the terror abruptly vanishes, and the person will not be afraid of that dragon again for 24 hours.
Given the differences from ordinary fear that one person can try to invoke in another, is it understandable how I think dragon fear meets the threshold I described earlier as "beyond the level of influence possible in the real world"? And that dragon fear therefore seems to me like an unrealistic "magic" ability even if it isn't technically magical enough to being stopped by an antimagic field?
To the extent that the frightened condition doesn't do a very good job of modelling fear, that seems to me to be a criticism of the condition rather than a reason to read any oddities of the condition back into a characterisation of dragon fear as categorically different from other scary things.