Thinking of it, QR codes make sense for when you buy a single-use ticket at the station with the intention to use it immediately.
We actually have this for suburban trains, it's just a receipt with a 1D barcode on it. You use the barcode to open the turnstile (on some stations where they are installed), but otherwise the tickets are checked by controllers that occasionally go through trains.
For getting around a city though, I don't see much of a good use case. In my city, if you're here for at least several days, you're expected to buy the refillable card. If you're only here briefly and only need to use the metro a couple times, it's 1.5x more expensive but you'd buy tokens or tap with your bank card.
We actually have this for suburban trains, it's just a receipt with a 1D barcode on it. You use the barcode to open the turnstile (on some stations where they are installed), but otherwise the tickets are checked by controllers that occasionally go through trains.
For getting around a city though, I don't see much of a good use case. In my city, if you're here for at least several days, you're expected to buy the refillable card. If you're only here briefly and only need to use the metro a couple times, it's 1.5x more expensive but you'd buy tokens or tap with your bank card.