Even if gamblers had an unlimited amount of money, they would still lose to casinos, because casinos have a maximum bet limit. Let's say a gambler keeps doubling bet size each consecutive loss faced. After a dozen bets, he won't be able to double them anymore, as that is the maximum bet size allowed by the house.

So they will have to continue placing maximum bet sizes or decrease them. As result, further losses will be faced, without possibility of recovering the money, since Martingale strategy isn't possible to be applied anymore from that moment on.

The system is fully designed to guarantee sustainability for casinos on long term, doesn't matter how hard gamblers try to overcome it.