Which makes my point get back: if Magic is Powerful and Can do Anything is the norm, it permeates everything. EVERY PC and NPC *has* to be a magic user, use a magic user hireling, or buy magic items from a magic user just to be able to survive. Hence, you can't build a NPC Pirate that mistrust magic, or simply that ignores it. He *needs* to know *what* a scry spell is becouse he *needs* to have a non-detection device beforehand.I will agree that this could be a problem, especially in slower-playing games, such as 3e. OTOH, if you follow the guidelines in 3e, almost anyone can gain some form of non-detection device that should prevent your (B).
When those consequences arise, yes. When they dont, no. Go back to my example: players might be a group of Robin Hood outlaws, so they might not care at all about their evidence been believed by the king. Once they know who killed the countess, they can act in consequence.The first sentence is correct (but, as a reply to the meme that it takes magical GM handwaving to deal with the problem, it shouldn't). The second sentence is wrong. Consequences to using magic, and magic not simply solving the problem (but adding a layer of complexity/decision points) does make magic less gamebreaking.
Yet if the local Inquisitor point the Count with his finger and shouts "Devil-Whorshiper!!", the Count is hanged. And that does not address AT ALL the fact of magic divination being a plot buster. At *best* it might be a counter for *that* example. The Players might be investigating who killed the farmer's wife, and discover it was the farmer (who has 0 political power). The problem is that magic divination crush plots. That the plot *might* be built in a fashion that even with magic divinations, players need other actions, only means Magic *forces* the DM to build some plots instead of others. No other aspect of the game removes the narrative control from the narrator such as magic does.BTW, in your examples of the "real Dark Ages": In the feudal system, the Count is the vassal of the King, and owes him both allegiance and military duty. The King is always making sure that the Count cannot amass too much power, while at the same time making sure that the Count has power enough to fulfill his obligations. Again, rather like the mafia in The Godfather.
The Count's ambition is always to raise his own standard; the King knows this, and the Count knows that the King knows this.
Unless the Count is a problem to the King, though, it is never in the King's interests to take him out. For this reason, in the real Dark Ages, as in any place in the world right now where similar conditions apply, the Count really can and does get away with murder. He doesn't even have to hide it; he just has to avoid broadcasting too loudly so as to become a problem to the King.
But I'm not talking exclusivelly about games (and thus a ruleset). The OP was talking about fiction. In Fiction, if magic exist to a level where it can "do anything", then everything else is pointless. Why would I hire a (mundane) assasin to kill a king? The king is going to be ressurrected, and will have magical defenses to protect himself. Only a magical assasin (with something like soul-trap) can kill him. That's why on most fiction (bassically, any that's not D&D based like Elminster series are), magic is non-existant, is a plot device, or do minor non plot-busting and shortcut effects. Gandalf did not teleport to Mordor. Witch King does not teleport to Frodo. Thulsa Doom does not Teleport to Conan. They have powerful effects (like casting circle of protection) than Martial Characters can't do. But those effects do ^not^ become into plotbusting, and is not far beyond of what a Martial Character can do (Glorfindel can't cast "circle of protection against Balrogs", but killed one with a club and a dagger, so the effect, while vissually different, isn't mechanically unique. It does not put Wizards beyond Warriors, given same level. Glorfindel is on par with Gandalf)I am not saying that people do not experience these sorts of problems; I am saying that people do not necessarily experience these sorts of problems. They are an artifact of the convergence of the ruleset and playstyle expectations, where the two do not harmonize.
Just to point: get to 3e rules. Add up the cost of a fleet of galleons, a bunch of camels, their sailors, drivers, some guards, and their food, for a couple of months. Now compare it with the cost of making a permanent "circle of teleport" from Florentia to China. And tell me why on hell would a rich florentian trader use any kind of mundane caravan to the Silk Road.