If ETH reached 1k, I'd pretty much lose faith in all cryptocurrencies' ability to innovate without everyone giving up on an entire currency every couple of years.
Immutability is a great characteristic but if ETH gets that far it'll show that decentralised innovation like Bitcoin promotes is not good enough to stay dominant.
ETH 1k and Bitcoin 4k would have ETH at a higher market cap than Bitcoin (not that market cap matters, but it's the only measure I can think of).
This! There are plenty of other currencies out there that offer much more useful innovations to the community than ETH. The main reason for ETH keeps growing is because it's in a bubble of it's own which will eventually burst. I don't think it will crash to it's low point of $7 this winter but it will see several major dumps before it gets to $1000. By then BTC will have long since passed the 4K milestone.