Other arguments I've seen against Bitcoins or even cryptocurrencies in general, are just flimsy imo. Cryptocurrencies will be a part of our future, there's no turning back now. The only question is which one?
This argument only works if you view Bitcoin as software. But in reality, Bitcoin is a protocol (like TCP/IP, HTML, DNS), and the difference is that protocols adapt because they're only the basis on which other software structures are built. We are still using protocols that were designed in the 60s and 70s, but what we have built on top of them makes today's internet unrecognizable from the networks of that time.
Just about the only argument against Bitcoin that has any credibility (that I've come across so far), is that governments could try and squash it by going after not Bitcoin itself, but the entities on the periphery of the network (exchanges, Bitcoin-based businesses and startups and so on). Of course, the counter argument to this is that there is no such thing as "governments" that act in a completely homogeneous manner. There are many different governments, each with its own set of priorities, and each in itself also in constant flux.
So if China decides to ban deposits to Bitcoin exchanges (to come up with an example), you can be certain that other countries will take a different approach. And, really, all you need is a handful of jurisdictions where Bitcoin can thrive.
Over the next 70 years or so quantum computing might become an issue, but then there will likely be consensus to change the protocol so that it isn't that big of a problem.