How's this any different than say, not bringing your wallet with you, because 95% of the time apple pay works, and then getting mad at the shop/credit card company/apple when the payment terminal randomly decides to not accept apple pay? Not to mention there are plenty of other ways a phone can fail. It can fall into a toilet, you can drop it and the screen cracks, etc. By not carrying the keys, you're rolling the dice every time.
>Just like I don't carry two car keys.
That all depends on your risk appetite. I normally wouldn't carry money in my socks, but if I'm in a foreign country I very well might. Likewise if I'm going on a road trip where I might be days away from home, I very well might bring a second key, in case i accidentally lock one in the car or whatever.