Or if you are worried about such scenario happening to the exchange you are using, better use binance as they have insurance for their traders. So even if they are hacked or something, you know, you can still get your coins. .
And you believe them?
If one of their cold wallets is compromised and they are forced to pay 5-10 billion do you think they have the money?
That's just another lie from the guy who runs an exchange that is not registered in any country, who do you think will insure those funds?
If they get hacked and they lose all BTC they have stored is goodbye for them and for your funds too.
You're asking questions that its like we are nostradamus or we are those bastards that will attack a website for ddos.
Not really sure what OP wants to hear, an exchange that survived a DDoS attack yesterday might succumb to another one tomorrow, a smaller one might never be a target as they would fly under the radar, and so on and on.
We're not talking about power failures that have clear back-up schemes, those are random and with random effects normally I would go the path of the bigger the better as they have more funds available to spend on security but that's no rule either.