Yes, willingness to write readable code is highest barrier for sure. And yes, I can imagine which kind of dumb code you mean - more like assumed in KISS.

However, designing a "framework" (or any place to write and organize the code) is a 10x harder task. (I have not even touched concurrent programming here!)

With the project growing, the pile of dumb code becomes like 1 + 1 + 1 + 1 + 1 + 1 ..., and at this point it really requires some intelligence to make it 1 x N. Because (general) intelligence is about recognizing and applying patterns after all. And it's not only speed of recognizing patterns. Otherwise 10 fools could do a job of one genius.

As for OOP and it's design patterns, I am coming more and more to conclusion, that OOP as it is usually understood only makes code more complicated. I am not hinting at FP here, just want to highlight that writing dumb code requires smart structure to "host" that code. And thinking through that smart structure is not for "dumb coders".

May be my personal bias shows here, but instead of spending 10 hours on implementing "1+2", "3+2", "4+6", "1+7", "4+5" user stories I go for "X+Y"... and use less than 10 hours for it. Win-win. Is it general intelligence or some other trait? I do not know. But I've observed a lot of developers who are happy to do "1+2", "3+2", ... and stamp "X+Y" as "too complicated"...

As for SOLID and design principles. I doubt anyone can learn those. I mean some "average" guy, not otherwise a team of superstar developers (which rediscover those quickly). Even the mere act of thinking from general principles to their instantiation is apparently not that simple. Judging from my personal experience and observations.