It's absolutely inevitable that Bitcoin will be replaced.
Proof:
https://blockchain.info/address/1CounterpartyXXXXXXXXXXXXXXXUWLpVrThe total Bitcoins that will ever be in circulation is now ~21M - 2114 at the time of this writing, just due to this one address. If I understand the concept, those Bitcoins paid to that address still exist, but are effectively unrecoverable. And this is just an example. People will lose backups, forget wallet encryption pass phrases, die without telling anyone about their wallets, etc..
I know there are like 2 quadrillion satoshis available, but that one counterparty address has eaten something like 200 billion satoshis in a very short time. That guy with his drive in the garbage dump (if it's real) blew another 700 billion. What if the FBI loses the paper they wrote their MultiBit password on? Another 20 trillion satoshis might be gone.
The math says there's virtually zero chance of recovering 'lost' coins (lost meaning the only way to get them back is by guessing), while there's a definite finite chance of losing the coins we do have. Therefore, at some point we'll need something new - granted, maybe far in the future!