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.
As of now there are hundreds of hundreds of bitcoin that have been discovered and there are still hundreds of hundreds coins that have not been discovered so I think someday for sure one coin will stand out and bitcoin will be replaced by other cryptocurrency, but for now let us celebrate that bitcoin is still on top.
The reason Bitcoin is still No.1 because it is better than Ethereum if we create a comparision the limit supply coin between Bitcoin and Ethereum. I know Ethereum is a good coin, it created by a young developer but has more benefit to apply in many fields than Bitcoin. But the fact, the value of Ethereum can not increase more because it has not a limit supply. Therefore, recently, Vitalik said he can makes Ethereum has a supply in the near future to improve this coin become the No.1 in the cryptocurrency world.