There are two problems. The first is that new technology (and I'm not talking about altcoin gimmicks, I mean ZeroCoin specifically) may introduce brand new, untested cryptography. What happens when you have millions of users and huge merchant adoption uptick? You can't play with people's money by opening them up to risk, so you stick with what is tried and tested.

The other problem is that it's not us who makes that decision. We may have some degree of swing now, but even right now - if we suddenly decided to introduce something stupid like doubling the block time without doubling the block reward, what would happen? Miners would ignore our changes, someone would fork the repo, and they'd continue development without us.

None of this means that Monero will eventually reach a point where change is impossible, it's just that some changes will be ill-advised until another cryptocurrency has demonstrated its value and cryptographic soundness over many years, and other changes may never live to see the light of day because miners will refuse to accept it. The upshot of this is that even if something like ZeroCoin is the holy grail and they've solved the obvious trust issue and they are absolutely certain no other exploits in the software exist (which will take years) it will still be a struggle for them to have any sort of adoption based on technology alone. There are WAY too many other variables at play.

 if this community was presented with a superior technology to ring signatures, I am certain that a hard fork would be made and we would continue to be the dominant community built up around the idea of an anonymous CryptoCurrency.