That's exactly how it works (well, you can use a debit or credit card) and in fact, if you're not a subscriber, a hold of 150€ is placed on your card until you

That's exactly how it works (well, you can use a debit or credit card) and in fact, if you're not a subscriber, a hold of 150€ is placed on your card until you return the bike. But I think they have to put an artifically low hold of 150€ on the bike (instead of the real cost of 400€) because far fewer could and/or would use it if that much money was held. A lot of the bikes are stolen from people who rented them (and in this case, maybe the person doesn't have to pay the 150€ if he/she can prove it?). Some are stolen from the trucks that are used to regularly transfer bikes across the city to even the distribution. Some people figured out how to hack the kiosk to give them multiple bikes for one payment (that has since been patched). Also, people can of course get them with stolen credit cards and by breaking the lock. You can see how the thefts can add up. I don't think the system is doomed though because people in Paris absolutley love the Velibs and want them to stay. I'm sure that they'll figure something out, whether it's higher rental costs, extra security or something else.