So why not just use fiat? Coinbase used to offer this service, where you would 'auto-topup' your balance, but removed this because it was silly as it generated multiple tx's just to do what paying in fiat would achieve in the first place.

As time goes on, the notion that coffee or meals are suitable to be paid in bitcoin seems more absurd. You would no more pay for a meal with a gold bar than you would with a Krugerrand. It will be seen as decadent for all those pizzas, controlled substances and ransoms to have been paid in BTC -- a kings ransom for pizza?

Bitcoin will likely never be a general purpose payment network. Lightning MAY if it obtains support from legacy services like VISA and MASTERCARD. I suspect V/MC will be the lightning nodes with the most payment channel volume.

So you're buying things with fiat currency.