> IQ tests are far from perfect and I think everyone agrees they are a woeful indicator of success.
I don't know many people that agree with this. At least in my bubble, the great majority regard IQ tests as bunk and the rest regard IQ as just a very roundabout way of measuring the education level of a child's parents. (There's some disagreement whether the father's level of education is the most important factor [1] or the mother's education level is [2]).) It's only in a select few internet forums dominated by a select few types of people where IQ is taken seriously and even obsessed over.
Putting aside the wholly made-up g factor, what we've been seeing in the West for the last 30 years is rapidly increasing inequality in educational attainment. Severe cuts in education at both the public level and the private level (see, eg, the extraordinary rise of single mothers in both the US and EU and parents with multiple jobs) means that many children are simply getting less education than they did 30 years ago.
Note that this does not apply to the knowledge elite who go to extraordinary measures to educate their children. It's quite likely that the children of today's elite are the most educated children in the history of the planet. I know a few parents who have budgeted a million dollars for the education of each of their children.
The Flynn Effect was likely nothing more than the result of the extraordinary and broad-based increase in wealth we saw from 1900 to 1980. Since 1980 virtually all of the growth is being captured by the elites so it's not surprising at all that we'd see a similar reversal in the Flynn Effect.
[1] https://www.theguardian.com/society/2014/sep/23/fathers-educ...
[2] http://bigthink.com/ideafeed/mothers-education-level-at-time...