He is not saying a "10x" language couldn't be invented, he is saying a hypothetical 10x language wouldn't increase overall developer productivity 10x.

His argument is developers use a lot of time on the intrinsic complexity of designing solutions for complex problems. Representing these solutions in code is a smaller part of the job. So even if a new language increased coding productivity by 10x (like going from assembler to C might) it wouldn't increase overall productivity with the same factor.

In short, the bottleneck is not the coding, the bottleneck is our minds thinking about how to solve the problem.

I reread the paper and I see what you mean — due to coding being only one part of the equation for productivity, any increase in that area can only speed up that part, not the whole (basically Amdahl’s law).

But it does also mention that since the appearance of high level languages, we are on a path of diminishing returns:

“The most a high-level language can do is to furnish all the constructs the programmer imagines in the abstract program. To be sure, the level of our sophistication in thinking about data structures, data types, and operations is steadily rising, but at an ever-decreasing rate. And language development approaches closer and closer to the sophistication of users. Moreover, at some point the elaboration of a high-level language becomes a burden that increases, not reduces, the intellectual task of the user who rarely uses the esoteric constructs.”

Also, Brooks originally wrote this paper for a 10 years timeline, and my point was mostly that even though we are like 3 times over its original length, I still don’t think languages would be even 3 times more productive, let alone more (and I won’t buy empirical evidence of your fav language, but some form of objective one).