I don’t think it will ever happen at all, but if it does, it will not be soon. In order for altcoins to supplant bitcoin, their dominance needs to grow above 60%, now it is only about 30%. But I really look forward to it happening.
Dominance is very important, but not all portions can be said to make bitcoin lose. Bitcoin apart from being one of the first cryptos known to the public, also brought altcoins to many users and investors. I'm not saying that bitcoin can't be beat, but the current position of bitcoin is almost unbeatable