I'm pretty sure it won't happened because as I know the great Altcoin that are on the most top was Ethereum and It also falls down it's value when bitcoins value also get down. That's it is not that easy to say that bitcoin can be defeated by any altcoins in the future.
Yes, facts prove that bitcoin is very strong with market dominance. However, all possibilities can occur, including the position of bitcoins that can be shifted by altcoins even though the probability is very small.