If bitcoin were to become mainstream, the transactional cost would raise because of the overhead with risk deferment.
Cryptocurrencies will definitely become mainstream because of the international and speed factors, but I don't think it will be bitcoin.
I see cryptocurrency use as being largely automated and transparent to the consumer in future. I think Bitcoin may have a place, but other more popular systems will provide solutions to many of its more gaping issues (rapid local settlement suitable for point of sale, complexity of implementation, lack of chargeback/cancellation channels, initial counterparty trust bootstrap, reputation management, etc.)
I started having a think about how these might come together over here ... comments/thoughts/collaboration welcome. http://ifex-project.org/
Right now, proponents of bitcoin like to tout the low transactional costs, compared to traditional payment systems, as a predictor of bitcoins's "impending" success.
Disputes, Chargebacks, etc. are handled now by financial institutions, at a cost to overhead. These financial institutions also have insurance to distribute their risk. If your account is hacked, you file some affadavits and you get your money back.
IF bitcoin were mass adopted, you can be sure that financial organizations that deal in bitcoin will tack on transactional costs relative to the risk of loss - almost like an insurance. In the case of Bitcoin - this is VERY high, unless of course the transactions are done with other financially certified organizations.
Aaaaaaaand we are back at a traditional banking system with transactional costs, but for bitcoins.