>>You could have multiple cryptocurrencies which support the exact same public API and clients could figure out their relative values automatically by crawling DEX markets (looking at current prices and daily trade volumes).
Transmitting between ledgers is associated with a cost, due to the capital requirements that make inter-chain bridges possible.
Look at the current attempts at such bridges, like Connext: they require individuals to run 'routing' nodes that faciliate inter-chain transfers by locking collateral in multiple chains to act as liquidity. There is a cost to this, that doesn't exist when two parties are transacting on the same ledger.
Transmitting between ledgers is associated with a cost, due to the capital requirements that make inter-chain bridges possible.
Look at the current attempts at such bridges, like Connext: they require individuals to run 'routing' nodes that faciliate inter-chain transfers by locking collateral in multiple chains to act as liquidity. There is a cost to this, that doesn't exist when two parties are transacting on the same ledger.