Bitcoin seems to be the past and Ethereum the future.
The question is : when BTC will worth almost zero? Next year? In 2 years?

I don't think so. Bitcoin will never be gone in the near future, since Bitcoin became the reference of all altcoins I don't think that ethereum can replace the position of bitcoin right now.
I agree with you hence bitcoin is the mother of all crypto which they somehow depend on it and no alt could able to replace it from its place. If theres a possibility then it would really take time and many thing to be considered first before it would happen as we all know that bitcoin is popular among others.