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.
It's hard to see Altcoin domination at 60% replacing Bitcoin,
Bitcoin is still always popular and Altcoins will always be behind Bitcoin,
Altcoin dominance being able to just 50% is good enough for me.