In Japan, because debit/credit cards have historically not been readily available, a payment system around transit cards has been built up. People put money on

In Japan, because debit/credit cards have historically not been readily available, a payment system around transit cards has been built up. People put money on to their transit cards and then can use those cards to pay for a variety of things from items in vending machines to MacDonald's, etc. In usage it basically works pretty similar to how the Lightning network will work and it's relatively popular. However, I think the OP is correct that if it becomes mainstream, 20M transactions a month will not be enough. Lightning requires 2 transactions -- one to reserve the funds and one to settle up at the end of the period. The transactions cancel if you do not settle up, so setting that period of time is vital. If you set it to a month (probably reasonable), then you are basically limiting the service to 10M users (assuming the OP's estimate of 20M transactions a month is correct -- I haven't checked). So I think it's pretty obvious that you are going to have to do something else