Every shining star has some drawbacks with it. In case of bitcoins hacking is the most sever drawback (apart from volatility). Bitcoin transactions are irreversible so yes hacking should be a nightmare for you.

I imagine that those who own a lot of BTC never leave on exchanges, or very little and very well scattered

I suggest using Trezor or Paper Wallet because hackers certainly will not be able to access it, unlike the cloud storage.

Do you think Ledger nano S is 100% safe? Do you believe if I plug my Ledger in an infected computer it will not hack my private key?
Just wondering around 

Precautions are better to get rid of anything is what I believe.
If we are careless then we are keeping ourself in trouble, I recommend you to maintain cold storage wallets, ledger Nano S, secure your account with 2FA verification authentication, avoid phishing websites, avoid public access and clean your history. Hackers always exists so prevent it by securing with proper authentication. Ensure you prioritize never clicking any suspicious links; it may come to you by email, forum, web, messenger etc. Do figure out which is phishing site before entering your info, user name, password. Check the domain, ssl very carefully.

You know how the saying goes, "better safe than sorry"! Online privacy and security should never be underestimated, even if you dont have a large amount of money invested in cryptocurrencies.

Always take care of your computer and wallet's integrity and try to avoid phishing and hacking attempts made by malicious users.

yes that is right you need to be much more careful, because hackers always remain as active to find clue how to hack your account, you need to have doubt security layer for your wallet which can give some extra protection.

No need to be afraid of hackers, they are just a bunch of useless people. Stay relaxed and always keep Private Key safe, so your bitcoin will be safe forever.

You havd to do something before your wallet is hacked because after hacked you can not do anything.
Make your wallet security complicated like 2FA, it will help to secure your wallet.

This is why people need to take care of their coins, people have hundreds of bitcoins but are using a pirated copy of windows to store their coins, it is obvious that money is going to get stolen, it is like leaving your wallet with a lot of money in the middle of the street.