That's very interesting, thanks for pointing it out to me.
In your system, why not just keep all the TXN's to Bitcoins.
When you put the money on an altchain - it just stores the amount of BTC you send. Not some other alt coin. There would be no rewards for mining except fees.
I would like it to be used to back sub-chains that get all of their value from the BTC-backing them, for example bitcoin substitutes intended to be used within specific geographic regions, but I think technically these bitcoin-substitute coins are still 'alternate cryptocurrencies', since they are not residing on the Bitcoin blockchain.
I think there's also value in making the proof of work transaction protocol flexible, and allowing people to design the sub-chains in any way they want, whether that means chains intended to be substitutes of bitcoin, or chains that are only partially backed by bitcoin, and are intended to be fully separate cryptocurrencies. Flexibility increases the space for innovation.
This system could be cascadeable. Altchain off an altchain off Bitcoin..
Well the sub-chains can have any rules they want, including having their own sub-chains. There are no constraints on how the sub-chains are designed, since the proposed proof of work transaction is validated only by the sub-chain block header.
The only bit I am unsure about is the Trust levels required.
You would need TO TRUST the owner of the sub chain. Since he could post an invalid Merkle-Root hash that assigned him all the btc. And cash out.
The owners are the miners, and yes you would have to trust them. If the Bitcoin nodes are going to validate the authenticity of the Bitcoin transaction the sub-chain miners are offering, then Bitcoin nodes would need to store all transactions in the sub-chain too, defeating the purpose of the sub-chains.