But that NPC will likely only be present in the campaign for a tiny fraction of time. That PC will be present (barring unforeseen circumstances) in every scene throughout the campaign. There's no difference in the scale. That NPC loses 100% of their agency, but, that's okay because you have so many NPC's. That PC loses agency, but, that's okay because you have so many interactions and time with that character.
Again, I'm failing to see the difference. In both cases, you're losing a "microfractional" amount of agency.
Again, nothing about this is forcing you to play your character in a way that is untrue to the character. It's that the mechanics provide the direction. How you interpret that direction is still up to you.
I'll admit though, I'm in the minority here. This ship has sailed a LONG time ago. There's no way that the fandom will ever allow this sort of thing in baseline D&D. Granted, in my current campaign, I have insanity rules, which means it's entirely plausible that your character will act in all sorts of bizarre fashions. And, I like games like FATE where you have Aspects and the DM can invoke that Aspect to compel you to play the character you created. Huge fan of those sorts of mechanics.
In my game we roll dice to resolve uncertainty. If I have enough detail on an NPC to know how they will react an intimidation or persuasion roll will not change their attitude. On the other hand, the miscellaneous shopkeeper, guard or other NPC that will only have 15 minutes of fame I usually haven't spent much time thinking about who they are. If I am playing a character and I'm uncertain how they would react I'll roll the dice myself.
Deception and similar rolls are not telling the target of the attempt what they think, it's to determine whether or not they see through the attempt at deception.