It seems that the OP was unaware that there is such a thing as bankroll management, yes, for casinos too, and limits on bet sizes. Nowadays, this can be done automatically; you don't need a human being doing calculations on paper to make sure the casino can pay out if a whale is lucky.
When it comes to casinos, especially large and reputable ones, paying out large winnings is generally not a problem and won't ruin them. Although, of course, casinos will come up with various rules, such as winning limits, to limit their losses.
That's right, the only thing that can happen is that a big win by a whale can cause a dip in the casino's bankroll. But these are usually one-off events. The casino pays out, continues to operate, and after a while, its bankroll is replenished.