the way it can "become" a currency is by merchants accepting it as a currency, simple as that.
we can already see many major stores, dealers, businesses, etc accepting bitcoin. that makes it a currency. only your disbelief makes this an argument
edit to add: anyone spending bitcoin on a car is actually an idiot unless they have the BTC to waste, in that case...
I can buy a shop and accept turnips as payment for my products, but that doesn't make turnips a currency.
There are not "many major stores" accepting Bitcoin as payment. There is an extremely small number of shops accepting it, many of whom did it just to get free advertising. It's ridiculous seeing a post on Reddit with 1000 upvotes which says "my mum's home business which sells figurines of cats made out of mouldy pasta and recycled string now accepts Bitcoin!" Who fucking cares, seriously? Who upvotes something like that?
I can only think of one place in my entire country (Australia) which accepts Bitcoin, and it's a pub in another state. So it means nothing to me.
Go look at the wallets for any of these shops. I bet most of them have had a negligible number of sales paid with Bitcoin. I believe the Baidu wallet only received donations from Westerners.
If I owned a shop I would accept Bitcoin, but ONLY because I know they're going to go up in value. So the money from someone who buys a $5 sandwich today may be worth $50 next year.
You have to remember Bitcoin is still in it's infancy though. While it's true that most places don't accept bitcoin, the number of places that do (both big and small) is growing rapidly. It's becoming easier for businesses to do such a thing, it's becoming easier for consumers to get and use bitcoin, and it's spreading.
If the world was to come to an end today, then yes Bitcoin isn't much of a currency. The world isn't coming to an end today though (I hope) , and the rate of places accepting bitcoin is growing pretty rapidly.
It wasn't that long ago though IT workers would have a hard time convinencing the company they work for that they should have a website. The common thing was "oh comon who uses websites, only a few people". Or remember back when online shopping first came about, it received a lot of backlash and as well and took nearly 20 years to reach the point it has today. So trying to remember bitcoin has only been around a few years and things like these don't happen over night.
I think it's very short sighted to think "well RIGHT NOW there's still not many places that accept it" and to come to the conclusion that that's always going to be the case. The trend says otherwise. It's not that far stretched to think more and more businesses will start to accept bitcoin and it will become more widespread, especially for online shopping/payments. Much crazier things have happened.