What I mean, is that you are better using a wallet on your pc then trusting some other random dude with questionable security.
You are making an assumption: that only you control your PC. Since at least 2006 or so, this is not really the case. Back around the year 2000, Microsoft rebooted the (Windows NT portion of the) Internet (even machines set not to automatically install updates).
The average machine runs Microsoft Windows. As part of good security practice, these computers often have anti-viruses and anti-malware installed. To remain effective, they need to be updated almost every day.
Web-browsers are almost Operating Systems themselves these days (running a variety of software-as-a-service). They similarly require frequent updates but for a slightly different reason: they are complex pieces of security-critical software. As a rule, nobody takes the time to prove complex software is correct (it takes about 7x the work if the L4 Micro-kernel is any indication).
If you want to install most software as a limited user: you will find that you probably can't. If the software requires administrative rights to install, it probably has control over most of the machine. Even software run as a limited user will likely have access to all of that user's files. You can mitigate this by running software in a Virtual Machine; but apparently that is not fool-proof.
When the price of Bitcoin hits $10,000 per coin: are all employees with access to signing keys at Microsoft, Symantec, Valve, Apple, Oracle, McAfee, The Mozilla Foundation, Google, Adobe, etc going to resist the urge to push out a wallet stealer to the user-base? Because the ELUAs you never read always encourage users to reverse-engineer any updates to make sure they a benign, correct?
Even if your are using a GNU/Linux distro, the bulk of your coins should be in "cold storage."
