Evolutionary bottle neck events don’t work in such a polite and calm fashion, and there have been many of them.

What happens is 90% of the population (or often more like 99.999%!) that don’t already happen to be carrying an advantageous mutation perish. Sometimes the whole species goes extinct if there is no such mutation already there.

The remainder, if there are any, then don’t have to worry about as much competition, and rapidly reproduce to fill the now mostly empty niche.