Of course, Bitcoin will inevitably fail when a newer technology takes its spot. But it'll probably be another crypto-currency.
Right. Just like the Internet failed when we replaced with Internet2.
I don't think any of us are using the same technology when the Internet was first introduced. If you're implying the Internet as a concept, of course there will never be an Internet2. And if we're implying Crypto-currencies as a concept, likewise. But specifically, considering what Bitcoin actually is right now (not even completely 1.0, mind you), we'll inevitably abandon it for something new and improved.
As masize said, it's incomparable. A lot of what the Internet brought over from its first incarnation is still around, but a lot of it isn't; HTML is great, but HTML5 is probably better than HTML2. I'm not saying HTML2 isn't around anymore, but why use HTML2 when everyone wants you to use HTML4 or HTML5? Python's great, but most everyone prefers Python 2 at this point. I also heard Windows has been steadily releasing newer OSes for a while; did Windows 98 disappear? No. Does anyone use it? I suppose. Software becomes obsolete as technology improves. Why should Bitcoin be the exception? It has certain hard-coded rules that cannot be upgraded without forking the whole thing. Now everyone's gotta use one or the other: let's say, Bitcoin 1, and Bitcoin 2. Bitcoin 2 has all the improvements over Bitcoin 1, so everyone moves their assets from Bitcoin 1 to Bitcoin 2. Inevitably, people stop using Bitcoin 1. Now: is Bitcoin 2 literally going to be Bitcoin, or is it going to be another form of crypto-currency all together? Either way, the original technology is left behind, and a newer technology takes its spot. Thus, Bitcoin, without anyone to trade with, as we've all moved onto another technology, fails; rather, fail is a harsh word, and I feel I should've picked a better one. Bitcoin will lose enough popularity for its value to drop tremendously from where it would've been if no superior alternative was proposed (including an improved, forked version of Bitcoin.) This could happen in ten years, it could happen in a hundred years, but at the rate technology improves, it is an inevitability. What I do believe, however, will remain for a long time, is what Bitcoin is: a crypto-currency. The concept of crypto-currency will doubtfully have a crypto-currency2; rather, it will simply be improved over time.