you realize that they dont have your private key right? and that you can verify the javascript wasnt poisoned by having a hash of the legit javascript and comparing it? with a handy toolbar?
what makes you think it's a problem? the fewer bitcoins in circulation, the more they will be worth. And being (theoretically) infinitely divisible means that the tiniest fraction of a single bitcoin would still be enough to go around if all other bitcoin was somehow lost.
keep your wallet file encrypted and back it up to multiple locations on a regular basis (to update the backups with new private keys that are created by your client software).
there are also ways of generating bitcoin keys completely offline as well as producing signed valid bitcoin transactions completely offline. This way you can forward funds to keys that are not on a machine connected to the internet, or keys that are backed up only on paper (in multiple safety deposit boxes if you like). And also you can then put signed transactions from the offline machine onto a usb stick or whatever and then use a networked machine to forward those valid transactions to the bitcoin network.
Coinbase is doing something like this for their storage of customer funds. Coinbase seeks to be a bitcoin bank that wont get hacked or that if it somehow does get hacked (cough inside job, cough) that only very small losses could occur.
could they not just run their services off Tor and thumb their nose at the financial authorities like certain other sites and services do, seemingly with impunity?