But those feral children are smarter than the trees that "trained" them. I didn't say that teaching has no value but it doesn't put a hard cap on what can't be learned.

Let's assume you are correct. IQ is not real but we can use it as a stand in for overall intelligence. If I have an IQ of then I can train multiple intelligences with an array of IQ but the top level is 150. That is the top though, but the bottom. So I can train something from 1-150.

The second key point is that intelligence is variable. We know that different people and machines have different levels of intelligence.

With these two principles we would see a degradation of intelligence. We can simulate the process by saying that intelligence has a variability of 10 points.

Generation 1 - start at 150, gen 2 is 148.

Gen 2 - start 148, gen 3 is 145.

Gen 3 - start 145, gen 3 is 135...

Since variation can only decrease the intelligence at each generation society will become dumber.

However, we know that in the past we didn't understand quantum physics, we didn't understand hand washing, and if you go back far enough we didn't have speech.

We know through evolution that intelligence increases through generations. For society it is beyond obvious that knowledge and capability in the world increases over time (we can do more today than we could ten years ago).

Your hypothesis is exactly backwards. Intelligence and knowledge are tools that are used to build even greater knowledge and intelligence. On average, a thing will be more intelligence than the thing that trains it because the trainer can synthesize and summarize their knowledge, pass it on, and the trainee can then add more knowledge and consideration on top of what they were handed.