This statement seems like it would be nice and easy. Yet, one does not simply wish away the complexity of managing any such system. To remove such complexity, one would have to either create some kind of grand abstraction, which is not what you were suggesting, or handle all of the complexity in the background using equally complex back end mechanics.
This is why computers were invented, simplification of complex systems. You write software that allows you to type up, print, fax, email, etcetera, a document you once had to hand write with a quill and ink for each individual copy you needed, then physically send them to their destinations. Abstraction is the lazy way of automating a complicated process. You skip the work of creating a resourcing structure. It's not any easier for the end user to manage unless it's just plain brain dead. It's automation that you need a computer for, how complex your automation is just depends on how much time you can spend coding it, not hot much time your end user can spend using it.
Balancing the current economy is hellish work. If you call bullshit, you're brain dead or full of it, one or the other. Trying to find the sweet spot just between materials and gildar is like pulling teeth. Perfect utilization is just wishful thinking, your whole dynamic changes with every upgrade. So you spend ten minutes combing through your options, current draw, and possible usage, poof, 20% more gildar from your top city, now you've got too many materials again. Back to the drawing board. It's a bleeding mess, you have no work flow.
A top to bottom production system has flow, you're balancing usage against income. It's vastly simpler to manage the base inputs as a result. You're not saving for 50 turns to build a horrendously expensive unit that takes all the wrong amounts. You don't have an end cost, you have end products. With a dynamic UI that shows the load of the inputs and outputs as you look at a context related item, you can immediately know the affect of building your new swords on their base components, and your supply of swords. Overdraw your production and you waste manpower at idle forges. Underdraw your production and you have iron stockpiling and fewer weapons and armor than you could have. Set up the proper UI for the task, and your nightmare scenario of hundreds of resources from start to finish is simpler than what we have now in terms of time needed to manipulate it.
Caravans aren't even complicated, they can be manually controlled and you can spend five hours out of a game futzing with them, or they can be automated. It's programming time in exchange for player time. The better they set up the automation, the less work it will be. If caravans automatically generate at need, you obviously no longer have to deal with creating them. If they have flexible pathing that can be pulled by mouse, control over where they roam is simple as well. You'll have less work creating them than you do now, as a result of not creating them. You'll have less work guarding them as well, because losing one wont be such a pain in the ass. You wont have to find the city it was taken from and replace it, you just have to hunt the looting bastards down and stop the resource bleed.
Is it possible for you to expand this some? For someone who has taken the position of the fun of making each and every sword used in the game, it seems moderately out of character for you to express such a non-realistic suggestion. What bandit "captures" a caravan? I was under the impression that caravans are generally burned to the ground after they are pillaged.
You only burn them down when you don't have the time to steal them. Burning someones supplies isn't nearly as good as capturing them for your own use. This should be self explanatory.
A "Torch it" button would be the most you'd need here. That's only if you want captured supplies to become their own enemy caravan that tries to run back home. Personally, I'm game for it, I'd like supply lines to feed armies above foraging density as well. Not because I like realism, but because it solves various tactical and strategic problems presented by stupid game design. No more steam roller super stack, no more need for stack limits, any of that nonsense. The more concentrated the army, the easier it is to cut the supply line off. The automated resourcing system need not actually do this though. You don't even need to be able to capture caravans at all, resources in cities on the other hand really need to be capture ready.