But I think the point is that minecarts can be automated -- they go downhill by themselves, or uphill with mechanical power. That means no dwarf needs to be involved in the hauling at all (only at the ends), nor would there have to be any pathfinding btw. because the cart simply follows the track...
One can already transport items downhill quickly by just letting them fall down.
That's harder to make work than you make it sound. You can do that by using a garbage zone, but then you can't use a garbage zone for what they're for, which is, you know, dropping garbage. And you can't do any kind of sorting with a garbage zone-- fisher berry seeds and large iron mail shirts will end up at the same destination. Setting up complicated mechanics, just to allow dropping with sorting, is already done, even though it doesn't work super well.
Lets just say you want to transport iron from the surface 100 z-levels to the magma and weapons and armor back up to the surface. You need dwarfs to load and unload the minecarts. They must work at exactly the same speed to avoid minecart traffic jams. A high hauler must not consider the deep hauling jobs or they'll walk up and down as they always did. And the whole thing must run so quick that it beats a dwarf with a bin full of armor walking up 100 z-levels.
One of the things that minecarts enable is physically separated burrows. If minecarts can jump, they can cross space that dwarves cannot. (And I betcha they can cross channels full of water too.) If they're reasonably automated, that means easy movement of goods between physically isolated dwarves. In other words-- that hauler, 100 z-levels down, probably doesn't even have path to the trade depot.
A traffic jam shouldn't lead to waste of labor-- it should lead to a delay in the chain. Hopefully, more minecarts at the depot equals more "fill minecart" jobs grabbing more dwarves, same as a stockpile. In order to be a time-saving device, the whole thing needs to be faster than a dwarf (I for one hope it is not!), but it's much easier if your criterion is that it saves
labor. If it requires two dwarves taking ten steps to move a single unit of goods, then it doesn't matter if it takes a year to travel 100 z-levels-- it has saved dwarven labor.