Just keep an eye out for larger black holes appearing with a greater gravity.
There are only 3 possibilities for currency:
1) Inflationary, see $
2) ~stable, see .
3) Deflationary, see bitcoin, the only case resembling a blackhole.
What would be a larger blackhole?
A different and better cryptocurrency.
I honestly doubt it would be possible for a different and better cryptocurrency to overtake Bitcoin. I can't imagine which kind of features this currency would possess which are so superior that people would prefer it to Bitcoin, yet somehow cannot be adopted by Bitcoin itself. Bitcoin is build on consensus and if there exists a serious possibility that it would lose it's value due to people abandoning it for something better, consensus would dictate that Bitcoin should adopt those features. And as the Bitcoin core developers are some of the wealthiest Bitcoin owners I'm confident they'll act swiftly should any threat emerge (as they did last week).
The limited Bitcoin supply is also Satoshi's greatest trick. He realized that if the block reward would never end, there would always be a risk that Bitcoin users would prefer a deflationary altcoin since that would be more rewarding to it's miners and holders.
I kind of agree. There's definitely something to be said for critical mass, and if Bitcoin gets big enough, it'll have plenty of momentum to carry itself through any feature advancements made elsewhere. Sometimes the king can be dethroned though... (MySpace)