First, to summarize the main argument:

Premise: "residents of many Western countries as well as high-income non-Western ones are becoming less intelligent"

Therefore:
- It is unlikely we would e.g. end up colonizing the galaxy, develop AGI, or sustain a population of billions of people if we are less intelligent, so our descendants may not outnumber us
- Humans may no longer become deserving of moral consideration if their intelligence declines to that similar to existing animals 

I think the main reason why it isn't discussed is simply because most longtermists don't accept the premise.