No idea.
Why do we need to determine if humans are getting dumber or not?
It seems to me, then, that your primary objection is not that IQ scores are inaccurate, but that intelligence shouldn't be measured in the first place?
Which makes me think that you don't want anyone doing research into whether human intelligence is changing at all.
Or can you point to any obvious improvements that result from having measured “intelligence” for the last 100+ years?
When my grandpa was a kid they would measure your skull and decide if you are smart or a subhuman based on that. Makes about as much sense as our current IQ tests…
What do you think intelligence actually is? What effects do you think it has when it goes up or down in mass?
Since I don’t know a way to describe what intelligent actually means I can’t tell if it’s good or not to have more or less of it.
For some this seems to be a very “finished” topic, but we don’t have any widely accepted definitions of intelligence, not even for “less complex” organisms