That's rationalization. It explains how, after the fact, the character could end up having made the same decision that the player made. It doesn't explain what actually happened, and how that decision was actually made.
Nothing actually happened, because it's all make-believe. The decisions was made for the character, by the player.
The real reason why the character cast Cure II instead of Cure V is that the player could see how badly injured the character was - in terms of lost HP - and knew that Cure II would be sufficient while Cure V would be overkill.
In 5e, particularly, the difference between using one slot and another could be invisible or merely qualitative to the imagined character casting the spell. So a cleric might notice that healing a scratch for Otis the Miller doesn't take much effort, but healing an identical-seeming scratch for Count Brass tests the limits of his power. Or he might just pray, and sometimes some people get healed fully and others don't (the gods - the game designers DM and players who imagine him and his world - work in mysterious ways).
If the player isn't making the same decisions as the character, for the same reasons, then you're not role-playing... you're just story-telling...
There's no 'just' about it. And, yes, role-playing in 'director stance,' is still roleplaying, nor does RPing in 'actor stance,' absolutely require the kind of ego-submersive method acting, that you imply. There is no character, it's imaginary, there is not 'for the same reasons' because only the player has reasons (or reason, or that matter). The player, no matter how talented and devoted a method actor, is going to have his own reasons for every decision that won't be the reasons he imagines for his character. That's inevitable.
"Games of the Imagination," yes. That's the whole point.
Any rule or mechanic which treats them like mere characters in a story, or like game-pieces on a board, is entirely missing the point.
On the contrary. A character in a story is a role. Pieces on a board are part of a game. That's what a Role Playing Game is.
If that's the goal of a story-game, then the rules utterly fail to allow that. The rules of FATE are written such as to constantly remind the players that these are characters in a story, and that they should do whatever is the most dramatic and "fun" at the table, rather than what the character would actually do in that situation.
That's the thing about fictional characters, they 'do' what's most dramatic or interesting or advances the plot or whatever. They don't
actually do anything. Because. Imaginary.
And real people in the real world actually do grow by experience.
They do. But not usually by the experience of killing things. I mean, you might get good at it, but if you're not a sociopath, there'll be other effects, as well. And, human learning isn't a one-way accumulation of experience, either. If you acquire a skill, you need to maintain it. Class/level is wildly unrealistic that way. It does protagonists in a story a little better, since they often face escalating challenges and are based on two-dimensional archetypes - but not a whole lot better, really.
I think it serves the game when more options are provided. Sure, some groups might not want to play with skills at all, but other groups might actually enjoy more detail.
I personally think it's more intuitive to make a Move Silently roll and make a second Hide in Shadows roll. The results of those two rolls apply directly to the situation with no extra interpretation required.
If you're using 5e's ability check system, though, you'll have to adjust the DC pretty dramatically to get anything like the same results, because needing to make two rolls instead of one to succeed is the equivalent of Disadvantage...
IMO, consolidation isn't always good for the game. For example, with more granular skills like "bartering", "oratory", "leadership", "diplomacy", and "story telling" a character regardless of class, can contribute to a negotiation in a unique way.
Problem is they also create incompetence. If the game has Diplomacy, you can invest in one skill and be a good diplomat. If has Diplomacy, Politics, Insight, Deception, Intimidate, Resistance, Conversation, Oratory, and Negotiation, and doesn't give you ten times as many skill ranks to spread around, it's going to be a lot harder to be a good diplomat.
Worse, if the list is open ended, you can build your character to be an great diplomat, but then when the DM adds Protocol and History skills on the fly, you're suddenly making rookie mistakes again.
Nope. 2e was designed that way for the purpose of backwards compatibility. Thief skills had been in the game since Supplement 1: Greyhawk. Non-weapon proficiencies using a separate subsystem were IIRC a product of Unearthed Arcana. They came from entirely different strands - and 2e kept them separate because that was the way it was done before.
NWPs might've been in the Survival Guides - or expanded by them. Also, presumably kept the Thief 'special' abilities not just for backwards compatibility but because NWPs were optional, and the Thief needed to be able to do Theifly stuff when the DM opted out.