Will another coin beat bitcoin in terms of total market cap? Maybe, maybe not. Is it possible? Very possible. Though Ethereum is slowly but surely closing up on market cap(currently a 57.3 billion difference[1]), it still has a far far way to go; and I honestly don't see bitcoin being replaced soon. Probably in the future, but who knows? Too early to tell. And it might not even be ETH.
[1]
https://coinmarketcap.com/Not only etherium that has chance to defeat bitcoin in term of marketcap.
Yes its true that its very possible to replace bitcoin in term of marketcap but not in price. Price of bitcoin is always become a leader of the other price. So its very difficult to defeat bitcoin