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/