> There's no way to instantly convert the stablecoin back into the collateral asset, you have to find someone with a CDP who is willing to buy the stablecoin from you.
This is incorrect and actually doesn't make sense. An important part of DeFi are DEXes (decentralised exchanges) such as Uniswap and Kyber. There is a liquidity pool where you can instantly buy/sell assets. There is no 'waiting' to find someone (i.e. no problems with coincidence of wants). You definitely do not need to find someone with a CDP. You can hold DAI without opening a CDP. Contracts can hold DAI (as DEXes do).
> Similarly, there's no way to unlock your collateralized asset, you have to find someone who is willing to sell you the stablecoin in order to open up your vault and get your eth out.
Also incorrect. You need to pay back the debt of your CDP with DAI. You can do this easily by buying DAI on a DEX, paying back the debt, then releasing your collateral. Some services exist to do this in 1 transaction, so you don't need to actually 'buy' any other asset. You just send the transaction to a contract and they take care of the details.
The DeFi, specifically the Ethereum space, has moved very quickly in a short amount of time, so there are a lot of new concepts and instruments out there. I think a more traditional finance person will have trouble understanding it all as in some cases, there are no analogies to the traditional finance system (e.g. flashloans).