I like this article from @nb: https://medium.com/galoymoney/lightning-as-a-retail-payment-system-part-1-7463c46342ef

If LN can eat all closed payment networks (Visa, Mastercard, Swift, ACH, etc) then the TAM is literally all of global payments. Because fees on LN will be so much lower, however, it's possible that value capture will be much smaller for companies in this world, which is generally a very good thing for people. The key in my mind, however, is that free global micropayments are going to enable orders of magnitude more commerce. When 8B people - most of whom currently have no means of conducting commerce online - can one day participate in the global economy, then the total pie of payment volume should skyrocket. So even if you're taking a smaller slice of each payment as a company, the total pie is going to be much, much bigger.