well, very much exchanger that became the target of hackers
It's not only online exchanges that are being tageted by hackers, but basically every site and service containing a certain amount of coins is a potential target.
maybe you're right, too dangerous to save all the coins that you have in the exchanger
It is in fact a dangerous move to store your coins in an exchange for long periods of time. If I take myself as an example, then I only have coins in my exchange account when I am doing some trading sessions. If I am done with trading, then I cash out everything. I don't leave a single Satoshi there.
but I guess exchanger has good security, so I still keep the coin that I keep as a long term investment in a website exchanger
You should not rely on, and take the "good" security of a certain exchange for granted. Nowadays inside jobs performed by employees or the exchange operators themselves, are an even bigger threat than hackers. That's why my advice is; If you're not doing anything other than holding your coins for the long term, then directly withdraw them to your own PC wallet. Don't put yourself in an unneeded risky position.