They will. Not all of them, but might be enough for a casino to go bankrupt. The algorithm for the whole process of a bonus program should be something that allows some gamblers to win big and withdraw, to spread the news, and, on the other hand, something that doesn't make the company to go over their promotional budget. It's good if the company is big enough to allow some unexpected expenses, but for a small company it can be a challenge.
I'm pretty sure that they would have a stop mechanism in place that would stop withdrawals if something goes wrong.
That has happened many times in financial institutions when they stop trading, so in a casino I'm sure they have similar mechanisms.
There's no way a casino will go bankrupt because of a bug like that.
Just stop? Without saying a word in advance? I disagree with such policy. I mean, bonuses are are there to attract people to the site. To make them register. If a site has a stop mechanism in place, like you said, they should state it in their ToS: "We reserve the right to suspend your withdrawal if something goes wrong through our fault". Without something like this it's not fair.
Big casinos are usually better than small ones in many ways, including the case you mentioned. I wouldn't recommend to bet big for anyone, but still it's better to play on big and recognized platforms.