I'm teaching macro right now
Given the specialization of various fantasy races it points to a trade system existing (and in mine it always has) even if it was not explicitly mentioned. Trade has always existed at whatever level transportation, production and markets allow for. It doesn't have to be ultra long range. Modern transportation does create larger markets and increase the benefit but the benefit has always been there. Medieval cities existed based on trade with surrounding villages, funneled through market towns. Long range trade tended to be in durable luxury goods. This wouldn't be any different. It means that the races would be much closer to each other geographically than modern nation states are. Dwarvish cities or Gnome steadings (or what have you) would rely on near by human or Halfling villages / towns for food. This doesn't mean Gnomes (and others) would produce none of their own food, just that they would be reliant for much of it on other communities. As for hidden Gnome villages they might just send caravans out to pick it up themselves or act through some trusted intermediaries. The ancient world supported a city of over 1 million people (Rome) on a highly organized system of long range trade with technology no better than the typical D&D world. Trade also offers numerous possibilities for adventure, which is another good, in game, reason for it.
Yeah, I was mostly thinking about it in terms of long distance trade, which as you say, tended to be more durable items. Shipping food stuffs over a multi-month journey with limited preservation technology is incredibly difficult.
Also, most DnD worlds have far deadlier wildernesses than our did, the chances of being accosted by dangerous elements is at least double.
But, reducing the geographic scale, and having a well patrolled and protected highway system would make things easier. As well as finding ways to use magic to cheat and make such systems more durable to fluctuations caused by weather and monster attacks.
I think most people here have pretty much hit the nail on the head with regards to the three classes.
When I saw the Inventor I was thinking Troibrand the Metal Mage but then the actual mechanics stole that dream away. I'm seeing way too much overlap with the classes to the be honest. It's like being able to do something in 6 ways that are very similar with only a small difference distinguishing them from the other.
I hope they haven't reached the limits to the mechanics.