There is a huge difference between a Bitcoin 'proxy' and a Gold proxy. Gox-Bitcoin, Bitstamp-Bitcoin, Coinbase-Bitcoin, those are all Bitcoin 'proxies.' Do those eventually obsolete Bitcoin? Obviously not, because merchants can easily accept Bitcoin itself, not the proxy, and thus the proxy is constantly being converted into the base currency.
Money proxies go through many stages. In the beginnings of gold-based fractional-reserve banking, although goldsmiths gave people receipts for deposited gold, those receipts were not yet money: people started using them as money because they were much more convenient than the gold they represented. Then, goldsmiths started loaning receipts for nonexistent gold. Fast-forward to 1971: Nixon eliminates the last vestige of gold-backing from the dollar. Do not get fooled by the seeming innocence of monetary proxies.
Monetary proxies exist because they serve a purpose. What do you suggest, banning them all? So much for the free market
Still, you are not just talking about money proxies, but about those proxies in the context of fractional-reserve banking. In other words, you already start in a relatively advanced stage of monetary proxy development. In that stage, fractionally backed proxies are loaned recursively at interest, so:
1. Their supply must become a multiple of the money supply they represent.
And how can you possibly protect yourself against this? Banning loans altogether?
Say you are an 'innocent' loanbroker. Mr. Money comes in with 10 BTC, and says 'hey, lend this out and pay me 1% interest.' So you accept the money, and lend it out. Is there anything fraudulent about that transaction? Then, another person comes in with 10 BTC, and says 'hey, lend this out and pay me 1% interest.' What exactly are you going to do, make them
prove its not the same money? Of course not, you're going to take it, and re-lend it out as well. And if it was the same money? Then you "created" 20 BTC, when Mr. Money goes up to another person and says "hey, I lent this 10 BTC 6 months ago at 1% interest, want to buy it from me for 10 BTC, you'll make that 1% as profit at the end of the year?"
Who exactly is at fault in this scenario? Mr. Money asked you to invest his money. Is that in any way fraudulent? Nope. You did what he told you to do. Is that fraudulent? Nope. Somebody borrowed from you, using the rates you stated. Obviously, thats fine as well. Then he spent the money, and the person who got the money, who had no way of knowing it was borrowed, invested it in turn. Is that fraudulent? Of course not. Then, Mr. Money sold your debt, while fully representing what it is, and a person bought it, because buying the debt
made logical sense.
Nobody did anything wrong. Unless you are for banning debt entirely, fractional reserve can and will happen in one form or another because people want to borrow, people want to lend, and money lent to you is an asset, an asset that can be bought and sold just like any other asset.
2. Even more money must be created to pay the resulting interest.
Okay, no. If I give you 10 BTC, you lend it out, and owe me 10.5 BTC, and get a borrower to promise to pay you 11 BTC, and somebody spends that 10 BTC again, and that 10 BTC is lent to you again, and you owe 10.5 BTC, and you get another borrower to promise to pay you back 11 BTC, that additional 2 BTC that you say must be 'created', will, in a free market, either be (A) earned by the borrowers or (B) destroyed, when a default occurs.
The only reason why in the USD system dollars must constantly be created to pay the resulting interest is because there is effectively no M0, and every dollar carries an interest rate payable to the federal reserve, who in turn passes a fraction of those profits to banks. There is no such constant 'leak' of value of all Bitcoins, because they don't carry an interest rate payable to anyone.
Because no more bitcoins can be created, the ratio of bitcoin to its proxy representations must continually decrease, just like that of gold to dollars continually decreased, eventually reaching zero.
However, we need not worry too much about all this because Bitcoin proxies have no inherent reason to be more convenient than bitcoins themselves.
Obviously they are more convenient because, of course, they will earn you interest if you invest your Bitcoins into them. Which means people, for fairly clear reasons, will do so.