Not only ETH is the key to the future because there are a lot of other promising alts like EOS and NEO. But ETH is the most valueable and its chances are higher.
I agree that Ethereum is now a very valuable coin, and competitors cannot yet catch up with it. But Ethereum has weaknesses that cannot be eliminated. And now really EOS and NEO, are the leading competitors, but there are new coins that have all the chances to become leaders Aelf, Seele, etc.
It is not possible, although Ethereum is the top Altcoin but there is a huge difference between bitcoin and Altcoin. The price difference is so high that Ethereum will take several years to reach to the current price level of bitcoin and then bitcoin will be more than $30000. Therefore in my opinion ehtereum will ever be a next bitcoin but will be the second coin.