> Is there a difference between your (c) hypothetical future ideas and hypothetical future scaling problems?
Hypothetical future ideas require development. Hypothetical future scaling problems block current development.
For example, suppose that Amazon Marketplace wanted, as soon as possible, to have all customers pay through bitcoin including through lightning network). The question is, would the average fee per transaction average 1c or less?
I claim the answer is no. Lightning channels require broadcasts (though much less frequently than 1 per merchant transaction) on the main network. Let's suppose someone does 100 transactions with amazon per 1 broadcast to the bitcoin network, which seems generous to me. A google search[0] suggests that the current average transaction fee on bitcoin main network is about 2.5 dollars, which then averages to 2.5 cents per lightning transaction with Amazon. This is close to the desired goal of 1 cent per transaction, under the listed assumptions.
However, Amazon adopting bitcoin for all transactions would drive a ridiculous amount of traffic to the bitcoin network. If we go to recent history, march-april 2021 saw about $20 average transaction cost (or more - peak at $60). Going back to our same calculations, $20 per broadcast puts us at a fee of 20c per amazon transaction, far beyond our goal of 1c per transaction. Furthermore historical data has not yet seen Bitcoin as a primary transaction network for a large commerce site like Amazon, so it's reasonable to speculate we could see per-broadcast fees average above $100.
In simpler terms, while Lightning network greatly decreases the fee per real-world transaction, it's not enough to handle large scale adoption unless ratio of real-world transactions to bitcoin broadcasts increase in direct proportion to the fees on the bitcoin network. Realistically, if a large amount of US commerce occurred on the bitcoin network, the most optimistic access pattern would have users broadcast to true-up their channel liquidity roughly in line with receiving paychecks - maybe twice a month.
The simple fact is that if you take millions of humans and have them broadcast twice a month on the bitcoin network, the bitcoin network would not be able to handle it (bitcoin volume is roughly 10 million transactions per month[1]). Forget even hoping to scale to billions of humans.
[0] https://ycharts.com/indicators/bitcoin_average_transaction_f...
[1] https://en.wikipedia.org/wiki/Bitcoin_scalability_problem#/m...