It's really just a simple waiting game. There is a strong need for a world reserve currency (currently filled by USD, and somewhat my EUR). Fiat currencies have a 100% failure rate, and an average life expectancy of 27 years. Even USD we use today is only about 45 years old, since it switched from being a gold-backed currency to free floating fiat in 1971. So as long as you believe that bitcoin, being an open-source free-to-innovate platform, can continue to exist and be innovated on top of, it should simply outlast all other fiat currencies one by one, with the countries of those failed currencies adopting both bitcoin and the last world reserve currency. If USD collapses within a few short years, with bitcoin adoption still being low those countries will adopt both bitcoin and EUR. If USD lasts for a few more decades, most countries will probably end up using USD and BTC already, and will switch exclusively to BTC as soon as USD dies. At that point, BTC will have completely replaced all fiat.
There's also the black hole fiat collapse scenario, where if bitcoin gains about 50% of world-wide adoption, where it will not make a difference for a merchant or person whether he receives USD or BTC, at that point BTC's technological advantages over USD will kick in full force, people will start preferring BTC, and as a result, the balance of use will tip greatly in BTC's favor. This also means that with more and more people choosing to take BTC, BTC adoption and price will suddenly skyrocket, people will start dumping dollars to switch to BTC, which will make BTC price go up even faster, which will cause more people to switch, over and over in a self feeding cycle, with BTC literally sucking out all the wealth from the USD in a very short period of time. The 50% adoption rate is the "event horizon" in this black hole scenario, and if this happens, it will happen VERY suddenly and VERY quickly, likely causing a whole lot of damage to people too slow to keep up in the process.
The worst are the people who have some political cause, claim Bitcoin is part of that cause, and try to imply that anyone who uses Bitcoin agrees with them and supports their cause.
As Andreas says, Bitcoin's political cause is neutrality, while the world's political cause is SEVERELY skewed to the side. So, in that kind of a world, just being neutral seems like being politically radical. Kind of like just being neutral to everyone and treating everyone equally was a radical political stance at a time when some people were slaves.