This is the golden rule when navigating your way through crypto land. If you don't have access to private keys or seeds, you'd risk yourself getting all of your coins lost or stolen in the long run. Most people rely on centralized exchanges and wallet providers because it's more convenient for them. If only they knew that without the keys or seeds they don't control their funds, they would've used switched to non-custodial or decentralized solutions already. We need to educate the masses in order to reduce attack surfaces as much as possible. As long as the majority has access to keys/seeds, the number of hacks will be reduced by a large margin.
When you don't own key of your coin, you must trust a third party with their honest, fairness and seamless operation for your transaction request. You don't control anything. What you see is a credit number in your account and whether your order for trading, withdrawal request is approved by third party or not, it is unknown and uncertain.
Nightmare might appear anytime if you don't own your keys.
At least, crypto/Blockchain tech's popularity is rising. Who knows if people become more aware about securing their funds in the crypto/Blockchain space?
I am sure that with two simultaneously trends: growth of Bitcoin (in value) and Bitcoin educational efforts, people will become more knowledgeable about Bitcoin and related things (how to store it, backup, recover, etc.) and less people will lose their big amount of Bitcoin because of unknowledgeable.
However, a second part is important. Being knowledgeable does not naturally and automatically cause good practice. It's a different side of a coin.