I don’t agree that this requires dismissal. Social skills in a monster’s stat block serve exactly the same function as social skills in a PC’s character sheet, and as non-social skills in either place: to inform the person who controls the character (the DM in the case of monsters and the player in the case of a PC) what subset of ability checks they can apply their proficiency bonus to. In the case of social skills, it’s ability checks made to resolve uncertainty in the outcome of an action made to socially influence another creature. There is no “using social skills on” PCs or monsters. There are only ability checks made to resolve uncertainty, which exists in the case of attempts to socially influence monsters/NPCs, and does not exist in the case of attempts to socially influence PCs.
Again, I don’t agree with you that determining what a character knows or observes is the same thing as determining what a character thinks. A knowledge check will tell you whether or not your character knows a certain piece of information. What your character does with that information - how they think and act - is up to you as a player. Likewise, a Wisdom (Insight) check will tell you whether or not your character successfully identifies another creature’s emotional state based on nonverbal cues. What your character thinks about that is up to you.
So far, I have not seen a convincing example of a place where the rest of the rules erode this argument.
Indeed, I agree. We must go into unsupported territory if we wish to establish a mechanic for the player to use to resolve their own uncertainty about what their character thinks. For example, we might establish that the player can ask the DM to make an ability check for the monster or NPC against a DC determined by the player. I actually think that would be a pretty good ruling. It’s not supported by the rules as far as I can tell, but that doesn’t make it bad or wrong. It just means you’ve got to do the work yourself cause the rules themselves don’t help you there.