I don't think it will ever happen. On Stellar there are several bank-like entities and each one print their #euro #usd or #btc token over Stellar, because they don't trust each other.
Bitcoin and other cryptocurrencies have value because are decentralized and permissionless.. not because an issuer says "trust my USD token".
If Ripple or Stellar became more expensive another network clone can be started to provide the same service.