If Ethereum mounted a successful 51% attack on Bitcoin, it would likely crash the value of Bitcoin, making Ethereum the new number one cryptocurrency.
Historically speaking, whenever Bitcoin crashed, it took the whole altcoin market down with it, including Ethereum. I highly doubt Ethereum would fare much better if it were the cause for such a crash (arguably,
especially if it were the cause of the crash).
The cost of a 51% attack has been estimated to be $6–$20 billion [1]. This is only 1.3%–4.4% of Ethereum's current market cap. And it is only 0.5%–1.5% of its potential growth if Ethereum knocks down Bitcoin and conquers its full share of the cryptocurrency market.
The same paper estimates an attack on Ethereum to cost around $34 billion. That would be around 2.5% of Bitcoin's market cap so the "threat" kinda goes both ways.