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.
I think you are not fully able to tell the problem and limits with the Bitcoin .
No doubt that Bitcoin has more fee for the transaction , and also mining of the Bitcoin is going down day to day daily , mine of one Bitcoin after mine of single Bitcoin is tough .
But still every people who enters in the cryptocurrency are only for the value , trust and popularity of the Bitcoin . And also they don't want easily enter in the other altcoins like eth , dash , dgh , ripple , minero etc because they have trust with Bitcoin and I also have only most of the trust with Bitcoin .
Here for me the eth is a altcoin which may be a good coin in the future to make trade but currently the Bitcoin is king and we can't do anything without Bitcoin , and in my opinion in next 10-20 years , Bitcoin will remain king .
If you are thinking about the limit of the numbers of the Bitcoin that 21M , then I will say that they will get spread in between the people in small small amount with big price value in the market thats all .