I've given a lot of thought to this lately.  However, the greater Bitcoin's market cap becomes, the less likely this is to happen.  Already, there's the equivalent of over 10 billion USD stored in the blockchain.  These folks aren't going to sit on their hands and watch their wealth get wiped out.  The best programmers and mathematicians will be hired, if necessary, to make sure Bitcoin remains the top cryptocurrency.  There's nothing preventing improvements in the protocol, as long as a majority of hashing power agrees with those improvements.  I think Bitcoin is pretty much unstoppable at this point.