about tree fiddy

Extremely slim. Bitcoin has serious known flaws. Future competitors won't.

FTFY

Satoshi's protocol when Bitcoin was first made available failed.  And so did subsequent revisions. On August 10th, 2010 [the leading implementation of the protocol] failed spectacularly even:
 - http://en.bitcoin.it/wiki/Incidents

But because the protocol can be modified through a certain process, the flaws are being solved (see BIP 30 as an example.).

Because the protocol started out as an experiment does not mean it hasn't or won't become a widely used and valued piece of our financial infrastructure.

Implied in your question is the assumption (hinted at in braces) that a crytocurrency can become mainstream at this point in human history. I don't think technology that a crypto-currency will rely one will mature for at least 7 generations (150 years or so). One thing that worries me almost as much as the possibility of hardware and software back-doors is bandwidth caps. Data and storage used by bitcoin is expected to increase at least geometrically for a long time. The average person many be excluded from caching the block-chain on that alone.

However, even if current technology is "good enough" to work in practice, it will be an experiment. There is always a risk that the big mining pools will get together and implement protocol changes that are detrimental to the currency. Even "bugfix" protocol changes are experimental. How far ahead such changes should be planned is an open question, factoring in the severity of the problem and whether existing transactions can still be trusted in the future.

This is also a political experiment. Governments around the world are increasingly cracking down on "annonymous" financial transactions. For example, the Bank of Canada has removed the $1000 bill from circulation because it is too easy to move around. Businesses dealing with Money are required to record who is sending money to whom. Bitcoin is (deliberately?) ambiguous. It is not really "virtual money" because there is no central issuer backing it. It also acts a lot like a commodity, which may have tax implications in many countries.

If space travel becomes possible, bitcoin will likely become obsolete. The protocol assumes that every node has a latency of less than 10 minutes (2 minutes if TCP is used). This can be violated on Earth as well if your Internet is cut off by the govenrment (or your ISP) and data must be smuggled out on foot. For the largest "island" of processing power, this is not a big problem. However, smaller islands of processing power face having their blockchain history re-written by malicous entites on the larger processing island.

The Euro is a large scale experiment started in the late 1990s. It's still in use in countries where it's a large scale failure.

What is your definition of "truly mainstream" ?

If 1% of the world uses Bitcoin for 1% of their transactions, then I'd still consider it a huge success. I think there's a good chance that happens, but that doesn't fit my definition of 'mainstream.'

I think there is small possibility Bitcoin proper is used by a majority of people in some country somewhere in the world use it at least once a week to pay for things. I'd consider that mainstream success.

I think there's a tiny possibility Bitcoin will eventually become as popular as the dollar.

But I'm not very good at predicting the future, so you might want to consult you local fortune teller.

Yes, more of a political experiment than technical

On the first draft of the OP, the modifier "truly" was not included, but opted to include for some unknown reason. I guess, at the time, I felt it read better. That said, thank you, Gavin, for your unique insight.

~Bruno~