These are not technical dangers that
Bitcoin possesses, they are dangers that exist with or without Bitcoin when dealing with money online. Computer security didn't all of a sudden become important because BTC came into existence: most of OP's same arguments could be made against having online banking:
* Online banking credentials are often stored in general-purpose personal computers running a wide variety of applications, managed by people who don't know how to secure a computer.
* In contrast, people regularly give complete access to their computers (where they manage their online banking, etrade, etc) to complete strangers by willingly or unwillingly installing software created by such strangers.
People have been being ripped off before BTC existed. The main difference with BTC is simply that a large amount of capital appeared very quickly in the hands of people that may not have been previously used to having a lot of money. If you imagine a hypothetical analogous scenario where .1% of online banking users suddenly saw their funds increase by 100x, and they didn't move those funds anywhere but instead continued treating them the same way, you could imagine them becoming fresh targets for malware or key loggers or phishing or what have you.