Anyone can learn to communicate in a simpler way, otherwise nobody would be able to train a dog. However doing so in most communications is not only tiring, but also has some pernicious psychological effects.

What would be the bad effects of using simple words to talk, despite being capable of procuring needlessly complex terminological constructs?

The problem is not about using simple words (which I totally support, BTW), but the speed and complexity of grasping new concepts. Having to decompose, simplify and slowly explain each element of a concept, vs. just saying the thing once and the other person building on it. One is tedious and makes you think everyone is an idiot, the other is an actual conversation.

It's not just having to use simple words, but also simple concepts, simple topics, etc.

The real problem is that talking this way is exhausting. As if someone told you that e.g. you are not allowed to use words containing the letter "m". It is certainly possible, but it requires constant attention to something other than the idea you are trying to convey.

Talking this way feels quite different from talking to someone who uses the same style as you.

I would say that it all boils to "topic relevance". What is relevant enough to foment communication is too divergent in different IQ levels. That brings miscomunication and specially boredom.

I see what you did there.

My writing coach always told me “Anyone can say anything if you give them enough words.”

Some people love to use big college words. I think it makes them feel superior. It’s hard to speak simply and clearly.