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.
Ethereum is learning from Bitcoin, and thats good. Problem for ETH is that it is just one coin in the osean of cryptocurrency. Usually people hear about Bitcoin and then understand that there is more coins: DOGE, DASH, Litecoin, and somewhere there Ethereum also..

Ethereum will fall .. and all other sryptocurrencies also. Ther is place only for Bitcoin!