Why is there is no point in adding more generations when an allele has become fixed

Low genetic diversity, which is seen with allele fixation, is dangerous as it can lead to mass extinctions. If there is little genetic variability within a population and the genetically similar individuals all are susceptible to a certain pathogen, the population will likely cease to exist. This is why we see examples of populations with fixed alleles becoming threatened or endangered

Explanation:

basically, it will cause extinction

the missing space is phylum