I thought for sure it would take until the end of 2018 but it looks like this summer ethereum might pass it up. With the turmoil around BTC potential splitting, it might be very soon. IMO this is healthy, we will always value bitcoin for being the one that started it all, but it's time for a more dynamic and adaptable currency to take the lead.
Ethereum is just a pumped altcoin and it does not have special features so that it could overcome bitcoin.But there are still some promising altcoins coming newly.So not today, but might be in future,bitcoin may be replaced by some other altcoin.But for that,it has to get the trust what bitcoin has got over these years among investors around the world and its not an easy task to achieve such a trust.
These days more and more altcoins are coming into usage with specific functions. Based on the same if those altcoins were compared to bitcoin, they might have an increased potential. The same potential when getting into reality won't be that effective as well cannot gain such a big support as bitcoin has gained.