Show me where the core assumption is that low level characters are the cream of the crop (unless you are starting beyond the first few levels). True, a first level PC may have a little more training than a non commoner NPC class, but I would not call a first level PC the cream of the crop as there are other people in the world equal to ( let alone more powerful) than they are. The only thing that truly makes first level PC special is that they are the hero/protagonist.
Also, can you tell me where it is stated that the core assumption is that the charcters are initially being trained to be professional adventurers. The classes are professions or vocations. How the characters become adventurers is an individual game decision based on the type of game/campaign the DM is running. If the DM is having them go town to town and the PC's are highering themselves out or if the PCs are seeking out various dungeons, then the professional adventurer title aookues. However, if someone gets swept into or ordered to participate in one big campaign expanding quest, I would call them adventurers, but not necessarily professional adventurers (at least not at the start).