IQ grew over the entire 20th century (Flynn effect). Even if it's declining now, it is credulous to take a trend over a few decades and extrapolate it to millennia from today. Especially when that trend of a few decades is itself a reversal of an even longer trend.

Compare this to other trends that we extrapolate out for millennia – increases in life expectancy and income. These are much more robust. Income has been steadily increasing since the Industrial Revolution and life expectancy possibly for even longer than that. That doesn't make extrapolation watertight by any means, but it's a way stronger foundation.

Also, I don't know much about the social context for this article that you say is controversial, but it strikes me as really weird to say "here's an empirical fact that might have moral implications, but EAs won't acknowledge it because its taboo and they're not truthseeking enough". That's putting the cart a few miles before the horse.

The meta level answer is that there are [many potential causes](https://forum.effectivealtruism.org/posts/SCqRu6shoa8ySvRAa/big-list-of-cause-candidates), and although suggestions are welcome, the "burden" on a new cause is high.

My best guess is that most EAs have never heard of this argument, and that the few who have heard of it think that it isn't accurate. It conflicts with their informal impressions/anecdotes, and it also conflicts with what they've learned about the world and society changing during the past few years.

If I did think it was accurate and really believed that human intelligence was getting worse over time, here are some thoughts that would probably come along with that:

• Does average/mean intelligence even make sense?
• Maybe we should do something about public schooling?
• Anything relating to restricting dumb people from having babies feels pretty unpalatable to me, so even if it was completely convinced that was the right choice, I'm not confident I would pursue it. I'm willing to accept sub-optimal results along some metric in exchange for having rights; I don't want to naively optimize.
• We probably shouldn't look at a trend of a decade or two and assume that it will continue in a linear fashion. (relevant XKCD)
• People were on average dumber 50 years ago (and 100, and 500), and human civilization still managed to survive, thrive, and develop. So maybe it would just be a yo-yo effect over the course of a couple centuries, and humans get smarter, then dumber, then smarter again, repeatedly? I can't say this with any confidence, but it does seem accurate that if the human average went down by 5 IQ points over the next few decades, it wouldn't cause civilization to collapse.
• Maybe this is somewhat similar to poor people having lots of kids, and as they get wealthier they have fewer kids? So maybe as people get more education they have fewer children also? So we should make sure that kindergarten and elementary schools are good quality.

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.