The lesson learned is that you should only have that much coins stored on an exchange you need for trades and IMMEDIATLY withdraw them as soon as you finished your trades. Exchanges may shut down from one day to another, happened to a lot of exchanges in the crypto space already.
Some people are just unteachable. A few weeks ago i saw a poll in one of the telegram chat: " where do you store your coins?". And what do you think? Almost half of those who have voted keep their assets on exchanges.
Still they are not learning their lessons. Maybe, they are waiting to lose their entire portfolio before they realise that they need to keep their coins in their own wallet. Big and famous exchanges are not free from possible hacking incident, so you should always keep your coins safe and secure.
Maybe they are too lazy or don't understand fully how to properly store assets. Anyway, nothing good will come of staying without additional efforts.