There's no downside to learning theory. I can't imagine why people think this.
One opinion that a friend of mine recently advanced, from his perspective of not-knowing-music-theory, was that highly trained musicians were boxed in by their theoretical knowledge.
Honestly I had a hard time keeping that convo going with him, because he also will on a different topic wax relentlessly about his training in engineering and how important it is, so I just kind of let that slide.
I think the confusion is that sometimes we run into a non-creative musician, a performer, who has tons of musical training, and they don't know how to just start playing off the top of their head. That is different than a creative person who has also learned theory and can both synthesize and accurately write down what they're synthesizing.
I've met and worked with both kinds of musician, and the fact is some people just don't have much adventurism, willingness to submit to the random, or whatever it is that makes a person generate fresh ideas, but still want to perform music, and there's not a thing in the world wrong with that. Just don't get wrapped up about whether or not you should learn theory based on how someone else behaves. That's silly.
Before I knew theory I wrote very odd songs that I awkwardly put together out of little bits of the patterns I was picking up from music I knew.
After I knew theory, it was like the difference between having one of those little boxes of 10 crayons, and the mega 64-crayon box or whatever that said you were damn serious about coloring. Many more options were open to me. When I compose today, years after my education ended, I have the tools to organize what I'm doing quickly, so I can finish a song more easily.