This is why you must take the responsibility that is attached to self custody in order to secure your wallet properly.
In such an ideal world, taking responsibility of self custody is not enough. Apart from privacy breaches, Imagine if you fall for a clone scams, the fraudster clones an e-commerce site and attach his personal bitcoin address to it and you buy a product from the cloned site, your coins are gone.
First of all, I think you have mistaken the meaning of "Ideal". In an ideal world, Bitcoin would not be stolen; there would be no scammers and thieves because an ideal world means a perfect world. It doesn't exist.
Secondly, the example you gave is the exact reason you must take responsibility for your coins. That is what self custody is all about. If you log in to a website, it is your responsibility to make sure it is an authentic website and not a cloned or fake site. Before sending Bitcoin to any address, you have to make sure it is the real address you want. Those are your responsibility. Those are things you have to do for yourself no matter what.
You can lose your Bitcoin in ways that there was nothing you could have done about, but they are very rare, like the recent coldcard situation. Going to a fake website, sending to the wrong address, losing your seed phrase, exposing your device or wallet, clicking of phishing links, and other similar things are things you have to be responsible for.