3 things:

1. No coin has the strength / hashrate of Bitcoin even Ripple couldn't move 1 billion dollars last year (they had to split it up into 3 separate transactions).
2. Bitcoin so far is the only coin that combines the 3 (security, speed, low fees). Other coins always drop one to gain the others. For exampe, dropping security to get speed and low fees.
3. An altcoin overcoming bitcoin would be detrimental to the market why? How do I put my life savings into something when it can be replaced by something better in a year or 2?