I think it is not possible for some altcoin to overcome Bitcoin. All of the altcoins are Bitcoin based. I mean, look at the exchange sites, how can you buy an altcoin? Of course through Bitcoin (and some via Ethereum). Aside from that, Bitcoin is the one who started it all. Therefore, it is the mther of all coins. And lastly, it is the only coin with unknown developers. We inly know its developer by the name of Satoshi Nakamoto.
Yes I agree it's impossible for other altcoin to overcome Bitcoin. It's like a tree, Bitcoin is the root and the altcoins are the branches. Without the root the tree will die.
It is not the same concept in bitcoin. If bitcoin will die then the altcoins will survive because there is already a lot of exchanges that accepts other altcoins as an exchange to other and not only bitcoin and the perfect example for that is the ethereum coin which you can use to trade altcoins in different exchanges.