Will ETH ever surpass BTC's market cap? I personally do not think so.
However, it is undeniably more useful compared to bitcoin in the sense that it supports smart contracts and whatnot, but that doesn't necessarily mean that it is a better currency per se.
Bitcoin is still the premier reserve currency in the crypto industry, and not a lot can be done to change that due to its first mover advantage.
Another aspect that makes impossible for ethereum and any other coin to surpass bitcoin is even more simple than that, all of them are using the blockchain, a concept developed by satoshi, so even if all of those altcoins have different functions at the same time we need to consider why a project that has borrowed so many ideas from bitcoin is going to surpass it? And the truth is that there is not a logical reason for this to happen.