I agree it would be convenient. But consider the new dangers: 1) pickpocketing would suddenly be 3x as profitable and 3x as easy, 2) lose one thing and you've lost everything, 3) if one thing stops working, everything stops working, 4) venues where you have to give up your phone means carrying extra cash in.... a wallet, 5) cybercriminals can now steal much more from you without ever getting near you, 6) I imagine there will never be a universal standard for these things so changing vendor will be quite difficult.
I can theoretically leave my home with just my smartphone. I can use it to pay fare at the local train station, get directions to restaurants, read the news, pay for food and coffee, and even lock / unlock my apartment.
I do still bring my wallet and keys, because I'd hate to run into a scenario where my phone does get lost, stolen, or damaged, leaving me stranded without a way to get home. But the concept is definitely there, and for someone less paranoid than I, it's definitely possible to live a life of convenience.
1) so lose the keychain and evenly distributed your keys in all your pockets to minimize the percentage of personal items lost per successful pickpocket