I agree.  For one thing it relies on encryption and a public log.  Encryption can be broken and with bitcoins as is, you can't force new encryption standards on old bitcoins.  As for the public log, if someone branches from it or introduces their own log then we loose credibility of the currency.

Bitcoins is a good proof of concept, but I think the concept will be taken over and improved upon by companies who don't respect the same level of anonymity that Bitcoins and Tor promote.