In theory it works like that but in practice it doesn't. Even in the Bretton Woods system there was a "notional" convertibility to gold. But there was still far more currency in circulation than there was gold in existence. The value of gold was simply pegged to a multiple of the dollar ($30 I think).

Lets say you had 1 bitcoin and you issued a bitcoin backed bond. You now have 2 effective bitcoins in circulation - the real one and the bond. The original doesn't cease to exist just because it's backing a bond. Similarly, crypto exchanges inflate the bitcoin money supply. We deposit our bitcoin on exchanges and they create these "synthetic" bitcoins for us to trade. Meanwhile the deposited BTC are still in circulation on the blockchain.

People tend to think that they're "locked away" and out of circulation, but they're not. The new synthetic ones are added to the supply. The exchange can do what they want with the deposits - it just depends on the contractural terms.

So the "21 million" limit is not really a limit. The bitcoin supply can be expanded in an unlimited way and will be simply through its use as a pure unit of account, same as any other asset.