As of now, there is no altcoin that can replace bitcoin. It had been the most successful crypto, people see bitcoin as the main cryptocurrency. It is too far from reality for ethereum to replace bitcoin at the current state since bitcoin dominates the market.
You are right, we are already seeing market capitalization for Bitcoin and how can people predict ETH will become 1st place. Bitcoin is always the dominating currency which it will dominate many years, ETH will always secure its the current position and it has no ability to dominate the Bitcoin.