> Again, this is nonsense. Exchanges between fiat and bitcoin can operate directly on the Lightning Network. There's no need to constantly open and close channels. You would send a direct lightning payment to your exchange to deposit money, or you would issue a lightning invoice to your exchange to withdraw money.
In this world where no LN-transaction touches the Bitcoin blockchain, what's the reason for having a blockchain in the first place?
The blockchain provides the security, ensuring that nobody can take funds unilaterally out of a channel. At all times, each party has bitcoin transactions for the correct channel state, partially signed by the channel's counter party, which either party can sign at any time to settle the channel balance on the chain (the settlement can be made to pay automatically into new lightning channels, although this is not implemented yet.)
It is like a credit line that can be closed by either party at any time, with the balance settled automatically by an impartial judge - the blockchain. Without the chain, we'd just have the existing system where you need human arbitration to settle balances if any party disagrees about their balance.
Bitcoin takes individual humans out of decision making about who owns what money. That's its fundamental purpose. We can build potentially any number of transaction systems on top of it, but it is not worth sacrificing the impartiality of the chain just to get a marginal increase in capacity of on-chain transactions. The chain's impartiality depends on the network being so finely distributed that no group can orchestrate control over it. This also requires that participants can be anonymous.
In this world where no LN-transaction touches the Bitcoin blockchain, what's the reason for having a blockchain in the first place?