Ethereum could break $1000 dollars today, if they reduced the amount of tokens in circulation by 75%. This would cause the price of a single Ethereum token to about 1000 dollars. However, this would only make calculating token value more difficult. There is currently 91,763,077 Ethereum in circulation. If, magically the price of Ethereum rose to 1000 dollars, Ethereum would be a 100 billion dollar company (almost a 900% increase from today's numbers.) In summary, it would take a hell of a lot of people buying Ethereum to cause the price to break $1000 dollars with the current circulating supply. I don't forsee Ethereum surpassing 25% of Bitcoin's worth per token.