7/11 is still 30-50% more expensive than the supermarkets, so irrespective of how affluent people are, it's a poor choice.
I used 'exchange rate' because not only is the yen weak, but the USD seems pretty strong - I guess it depends on where in the US you are from, but as a Brit, US feels expensive to me, Japan feels cheap, ergo Americans must find Japan even cheaper than I do.