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.
My advice is not to overthink things because they won't come true in the end.
I know there may be some people or those altcoin enthusiasts who will not agree with what I said but in the end don't compare kings too much with followers because in the end things like this will only make you dizzy yourself with various kinds of speculation which in the end actually weakens your own confidence in what you did before.
Bitcoin is still bitcoin where they will still be king and altcoins will never touch bitcoin apart from following its movements including for altcoins.
We can see how ETH is moving right now, is this not the answer for you when you see the movement of ETH and bitcoin now? I don't want to suggest or make it seem as if bitcoin is better but the evidence now speaks.