The design team of the Kin either could not envision or stubbornly ignored a world in which true smartphones are sold in Walmart for $100.
Once the decision is made to buy an internet-capable phone, the main cost is the service plan. Most teens can make up the difference in the fixed cost of the unit through a part time job, a birthday gift, or daddy's money. After the price of the 3G was lowered to $99, even these small differences in cost became nill.
The Kin was a ten year old concept, executed with five year old technology, and competing against gadgets from the future. Put simply, it was a mistake, executed poorly.
In any case the market still exists but more importantly is that the sidekick had instant messaging, apps, games, etc. You cant sell to kids and not have games.