Well IMO playing a musical instrument is doing something... it's the how and is just as important if not moreso than the intended result...
Its no different than the combat system, which weapon you swing is simply a detail, the weapon is the vehicle and the attack and its results are the important thing. You don't use a separate skill to attack with each weapon. At most you get a small bonus to attacks with specific weapons you are especially adept with. This is no different.
Now addressing your examples, If you're trying to influence someone but you're not a trained Diplomat (which encompasses a wide range of talents) but you can play extraordinary music that moves them, why would you roll Diplomacy? Why would a musician or even someone who plays as a hobby have to be a diplomat in order to influence someone through music... that makes no sense.
Why wouldn't you roll Diplomacy. CHA is the stat that deals with relating to other people, and this is a form of relationship to others, there's no better thing than a CHA check, and (at least in 4e) any CHA based skill is a CHA check, potentially with some modifiers. This fits with the concept that a limited skill list is a set of MOs, the character's approach to doing things, not a list of very specific knowledge.
Next your example goes even wider to Cha if I am trying to inspire them through music... so now instead of a musician I must also be naturally gifted in all ways of interacting with someone as well as naturally charming or commanding... why? There are plenty of examples of musicians with songs that move people who aren't necessarily any of those things. Then were back at Intimidate for trying to scare someone with a song... Again this is about how well I play an instrument not how threatening, hostile, etc. I personally am... or are the scariest songs always written by the scariest people?
Again, these make perfectly good sense, this is what you are accomplishing. The system is about supporting characterization. Still, who better to convey threat than someone adept at making threats? It all makes perfectly good sense. I guess you could turn them all into Insight checks if you are really determined, but Charisma really IS about connecting to people. That's what it is for. Does it REALLY matter what instrument you play or how well you play it? Is there really any doubt that a proficient player will play proficiently? Its not like this is a test of whether you're good at playing or not, your character can be as good as you wish to be, it is a matter of if you are EFFECTIVE.
Now swinging back to 4e... IMO it's not a feature that my character can't just be a musician by knowing an instrument or two but instead must be a master at Diplomacy, Intimidate as well as adept in Charisma to play anything inspiring, or that invokes emotions in others... YMMV of course and apparently it does. I also don't see how writing down "fiddler as a background in 4e is any different than 5e since in neither system is that an actual background with any type of mechanical backing to it...
Yes, to be successful you have to have good Charisma. Is that really not logical? What other characteristic of your character would be tied to success in a social endeavor?
4e allows you to construct 5 background elements from different categories for your character. Its an open-ended list, you can add anything you want, and the official background elements include plenty that reasonably allow for playing an instrument (including things like profession: musician). I'm not as familiar with the 5e backgrounds, but surely some of them now grant 'tool proficiency: instrument' do they not? How else do you get that?
I disagree... Intelligence measures mental acuity, accuracy of recall and the ability to reason... Only one of those is covered by the Investigate skill... ability to reason so I don't see how an overall increase in say memorization maps to Investigate but this would apply to an increase in overall Intelligence...
I don't think you can isolate specific mental skills and say only certain ones are good for doing something. Sherlock Holmes IIRC seems to have a very good memory, accuracy of recall, reasoning, and etc. In fact realistically the most critical factor in ALL these things seems to be short-term working memory, which arguably is the lynchpin of high intelligence. In any case a better memory would certainly make you a better investigator, IMHO.
My point was that it's not as crystal clear as you seem to be claiming...
I claimed that 5e's skill system was muddled in parts, you came back with 4e's system was too and gave this example. I referenced the compendium for all of 10 seconds and found a definitive answer. No, it isn't AT ALL UNCLEAR in 4e, you get a +2 bonus for using thieves tools, period, end of report.
What exactly would a "locks" or "traps" proficiency do that another proficiency or the tools don't cover?... I think the most important reasoning for the the tools proficiencies in 5e is because they make it clear that tools are necessary to perform the actions covered by them... I'm not a fan of the Fonzy school of picking locks.
There can't be any knowledge or understanding about locks and such things? Talk to a locksmith sometime. Sure they know how to open locks, and its a fairly significant part of their skill set, but it is FAR from all there is to the subject.
I thought you said you could produce actual quotes??
Dungeoneering...RC P.143
The dungeoneering skill represents knowledge and skills related to dungeon exploration, including finding one's way through underground complexes, navigating winding caverns, recognizing subterranean hazards, and foraging for food in the Underdark.
Training in this skill also represents formalized study or extensive experience . Those that have training in the skill can also identify creatures of the Far Realm...
There is nothing in this skill description about psionics... and this is from the RC, which was supposed to be an updated rules reference produced after PHB 3...
We're talking about what skill clearly is associated with what. 4e associates psionics with the Far Realm (and thus with monsters having the Aberration keyword), which is a clear part of the system and makes Dungeoneering the go-to skill for this kind of thing. Its pretty clear, even if its not relegated to one specific sentence somewhere in the rules, ask ANY 4e person on this thread, they'll all tell you the same thing. That didn't happen by accident.
But the whole point of your argument was that these skills and what they covered were precisely spelled out...
Please provide a quote because I am looking at the write p for the religion skill and it doesn't mention the Shadowfell or shadow magic at all...
Rules Compendium page 130. And I'm sorry, 'shadow' falls under Arcana, although the related topic of 'undead' falls under Religion.
"The Religion skill encompasses knowledge about gods, sacred writings, religious ceremonies, holy symbols, and theology. This knowledge extends to information about the undead and about the Astral Sea, including the creatures of that plane."
Again I am not seeing this precision you were claiming...
I don't know about simpler but it certainly would have backed up your claims of precision much better...
4e breaks all areas of knowledge down using certain keywords and topics which are applied to creatures, as well as any other objects as desired. The inhabitants of each of the other planes of existence, and each of the major types of monster, are allocated to one of the knowledge skills. In EVERY CASE where such a topic exists you can determine which knowledge skill is supposed to be primary for that topic. Obviously its possible to introduce material outside of that design, and its possible to introduce later material that isn't referenced within the skill descriptions presented in EARLIER material, but the design is quite clear.
Religion deals with the Astral and Divine, as well as the Undead. Arcana deals with the planes of existence more generally (all but the Astral and its domains). Dungeoneering covers the Far Realm. Nature covers the normal world in as much as something in it is natural/primal. It really is pretty darn clear and you can map every single creature to a knowledge skill for a check, and every plane. How much clearer can it get?