I have being giving this question a thought for days now, if other altcoins can be a threat to bitcoin especially ethereum which some folks are saying is a threat to bitcoin, and I came to understand that, the reason why other altcoins can never be a threat to bitcoin is that, bitcoin being the first and largest cryptocurrency has survived external attacks and attempted ban from government, which no altcoin has face such attack, because bitcoin has already paved way for them.
Op bitcoin is not threatened by any altcoin, for you to say bitcoin is threatened by any coin it means the coin is equivalent to more than half of bitcoin current price probably close to bitcoin, then the battle of becoming the most valuable crypto can be drawn. But since there is no coin that's close to 50% of bitcoin worth, all cryptocurrency are very far from comparing or seeing them as threat. Ethereum which you mentioned is no where close to the price of bitcoin irrespective of how long it has existed in the market, that shows that it'll be difficult for a project to pump out of no where to stand as a threat to bitcoin. Aside price of project the reputation of bitcoin is far greater than any altcoin or shitcoin in the market.