bitcoin cant replace fiat... transactions are too slow or you need to use a gateway.
if you want to buy something for example with two android wallets you have to use a faster altcoin
transaction are instant, confirmation are slow, don't confuse the two, if merchants can accept TX with zero confirmation, from for example off-chain(at least for small amount) we are done
you don't need faster confirmation for big purchase, i mean i can wait 1-2 hours if i want to buy a 32'' monitor 4k....