Therein lies the problem. Everything you mentioned can be solved if agencies have to take on a level of risk when they come up with a new product (a new model). In the current system, agencies only need to front up a little bit of money, with near guarantees of getting it back and then some in terms of return. It's practically loan sharking.
It's also why you see this assembly line of new faces. There's minimal risk to the agency if their model doesn't go anywhere. They could sign as many people as they can fit on their bulletin board. Meanwhile, none of the agencies want to be caught with their pants down when a new star is born, so they throw the widest nets they can into the ocean and hope to find one pearl among the fish. This system benefits everybody except for the most disenfranchised group (the models themselves).
The second you introduce more legitimate financial risks to the agency for mismanaging their models, then you'll see these agencies start to cut back and really try to find and push very specific faces. That's when you'll see models actually develop more lasting and legitimate careers because the agencies have more at stake if they are to fall by the wayside. However, short of models unionizing, this doesn't seem very likely. No one speaks for the girls, and agents and designers are allowed to do whatever they like.
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