Will be replaced by the next great innovation and will watch their market share dwindle down to nothing.
MySpace is a company, Bitcoin is a protocolThe internet is a protocol, nothing similar to the internet preceded it and nothing similar will replace it. The protocol will simply be updated.
Same will happen with bitcoin, the protocol will be updated where necessary and nothing needs to replace it.
The infrastructure around bitcoin is too large to just be replaced every few years, it took years to build and it's far from complete. No way merchants, banks, exchanges etc. will change to the newest altcoin all the time just for some random functionality that bitcoin doesn't have but the altcoin does or claims to have, it will be far too costly, it's far easier to just update the bitcoin protocol and no infrastructure needs to be changed drastically.
I think you can go even further. Bitcoin is both a payment system (nodes, miners, protocol) and a ledger of stored value (distribution of wealth encoded in the blockchain). Even in the unlikely event that something was found lacking in the payment system at some point in the future, people wouldn't tear up the ledger at the same time. They would instead use this ledger to boot-strap the new payment system / currency. The average user would only be lightly inconvenienced.
The blockchain is an unforgeable global ledger with 21 trillion "spots" on it. Each spot is 1 bit. You can prove ownership of these spots by producing an ECDSA signature with your private key, and you can reassign these spots to different people the same way. People are willing to trade real goods in exchange for spots on this ledger because they know someone else will trade them goods at a later date in return for those spots. So these "spots" on the big global ledger are a universal tool that makes bartering easier (they solve the double coincidence-of-wants problem).
When viewed from this perspective it becomes more clear that the "newness" of the blockchain is actually a negative. In fact, as it becomes more "boring" it actually becomes more legitimate and harder to tear apart or abandon. When we finally stop even thinking about what those spots on that unforgeable global ledger are, and we just accept it as "money," then it will be as entrenched as it can be.
The only reason the economic majority would want to abandon the dominant global ledger is if it was no longer perceived as legitimate. If it becomes clear to the people of the world that the distribution of wealth encoded by the blockchain is hurting rather than helping mankind, then they will abandon it for a new ledger. This would be a sort-of jubilee mechanism. Since bitcoin can still grow by at least 3 orders of magnitude and there is very little bitcoin-denominated debt, something like this would be a long long ways away.