[sph]

Use as currency is possible with Layer 2 systems like Lightning. The issues I believe are both technological, political and philosophical: apart from the ease of use (or lack thereof), people prefer the currency they need to pay taxes with, and will always prefer the depreciating one, until it stops having any useful value. See the arguments put forth by Saifedean Ammous in his book.

For me the main ledger is equivalent to inter-bank settlement payments. It is not for people to use directly, it is to settle the large amount between "market makers", so $10+ transaction fees are not a problem when the amounts transferred would be in the millions. These settlements are few and far between, they do not require immediate execution. In Bitcoin parlance, these market makers would be Lightning nodes, which I imagine in the long run would not be operated by your Average Joe.