Gold has to be "converted" to paper to be practical. Bitcoin is already a whole lot more practical than almost any aspect of the modern money infrastructure, so there's no "killer app" for Bitcoin-backed currency.
Banks extend credit money which is backed by reserve currencies. Money may start out as value-based (gold, bitcoins) but eventually a whole credit based economy springs up around it, and we're just where we started.
The "killer app" isn't being able to pay someone quicker. It's going to be the ability to borrow money!
(Although paying someone quicker is already the main reason two people use blockchain.info or something similar to transfer money instantly instead of waiting for a few confirmations. In small amounts you'll see "banking" like this, and then those banks will start lending money on the reserves they have.)
In other words, the same cycle always happens:
1) A value (scarcity) based currency is widely acepted
2) People store it when they aren't using it. They want to earn returns on it.
3) It is lent out by the "banks" to others, keeping a fraction on reserve
4) Governments regulate these reserve requirements and set up central banks which pay interest on the reserves
Now with Bitcoin right now the biggest thing stopping it from being used as a currency and 1-4 to happen is th massive price inflation. Who wants to spend a bitcoin when they can hoard it? Lots of people but apparently more people want to hoard it. Which is understandable because metcalfe's law is going to make ALMOST ALL the cryptocurrencies grow in value.
The good news is that this will make even more merchants accept bitcoin and once its price settles down (it might reach a saturation point only when many many merchants accept it, would be at least $100k a coin by then) then it can be used as a real currency that people really spend.
But then 1-4 will occur!
Sorry, you don't know how a bank works. The bank will borrow from someone else - depositors, other banks. The reserve requirements is how much of their own money they need to have to cover loans, and is a protection against risks, it does not imply that the rest of the money they hand out have been created out of the blue.
Yes, they will collect based on leveraging their capital, borrowing money at lower interest, and profiting from the multiplier.
But as to your scenario where the bank just hands out papers saying "here's 10BTC": In fact, you can already speculate on margin, on Bitcoin by buying Bitcoin CFD's (Contracts for Difference). CFD's means you're never actually owning the underlying security at all (and in fact there's no technical reason why the company you enter a contract with needs to ever own any at all either; whether there are legal reasons may depend on jurisdiction) - you're just, as it says, entering into a contract where you're putting up some cash now to be paid the difference in value over a time period.
I have an account with a company that would me buy Bitcoin CFD's with a 15% margin if I'd like. Meaning for $15, I "get" $100 worth of "here's some bitcoin" "paper" - if those $100 worth of BTC increase to $110, I can sell and get $25 back. Conversely, if they drop to $50, I'd get a nasty margin call to pay the $35 difference.
(yes, CFD's are high risk)
Bitcoin's scarcity is based on a completely fictional and arbitrary hard limit of 21 million which can be changed in a trivial fork such as Litecoin.
Furthermore, given that there is no intrinsic physical difference between a Litecoin, Freicoin, Bitcoin etc. there is an argument that a P2P currency is the ultimate fiat. The printing presses have just been handed to the public rather than being held by a select few.
Bitcoin is a great electronic currency, and a real challenge to money transmitters like Western Union, but it is not a store of wealth (in my opinion).
Limited resource physical commodities vs. a few bytes on your filesystem? This is a ridiculous comparison.
But what people mean when they say you could fork libcoin or something like that is not that Bitcoin isn't scarce. It's more along the lines of "We are in no way beholden to this psychotic black market pyramid scheme currency. We can just start anew with a better dispersement/capping scheme."
Referring to the bytes that constitute a valid block in the blockchain.
What distinguishes one blockchain from another? Nothing, they're just numbers.
What distinguishes gold from silver? Nature.
You can store your wealth in any commodity. I can store my value in stocks, gold or BitCoin to hedge against inflation. Just because something doesn't have physical intrinsic properties does not mean it somehow stops being a store of value.
Anything that someone wants to buy and someone wants to sell, that can be stored and transferred, can be a store of value.
Changed in a fork? Yes. Trivial? Not even close. Remember, you have to convince a lot of people to go along with your fork, due to the network effects required to get a new currency off the ground.
Does anyone remember the million dollar page?
As to being a fixed currency there would be a lot more bitcoins in circulation than actually exist as soon as banks started lending money and no you don't want to try and keep 100+k USD worth of bitcoins secure by your self it's just a fucking bad idea. Which means banks which means loans which inflates the currency. Of course because governments can't print money to bail out banks when they fail you just lose your money anyway and they will fail.
PS: Non private wallet = bank. The reason we don't see loans is because people don't want to borrow money in bitcoins.
The bank would have a database somewhere saying "We owe btbuildem 10 BTC". When you make a withdrawal they would take some out of their massive stock and hand them to you. So long as they have enough to cover withdrawals at any given moment they don't need enough to cover if everyone made a withdrawal at once, any more than they do with dollars/pounds/etc.