Are they claiming that the only value in solving these problems was for their field's personal development process? I thought Navier-Stokes (and some of the other millenium prize problems) actually had implications for useful technology. It would be insane to demand that people avoid making progress on technology that can save lives or improve general quality of life, just to protect the sanctity of your karate belt system. Perhaps in lieu of open problems left to solve, mathematicians should be welcome to take up chess or sudoku to keep their minds spry.
No, there's most likely zero practical benefit of having found a pathological edge case in which the Navier-Stokes equations do not work. We're most assuredly not talking about "saving lives" here. Unless advanced aliens show up and tell us they'll destroy Earth unless a counterexample to the N-S equations is provided within 24 hours.
> I thought Navier-Stokes (and some of the other millenium prize problems) actually had implications for useful technology.
This is the core misunderstanding that the open letter is attempting to correct.
Developing a better understanding of the Navier-Stokes equations could have a number of implications for useful technology. They're fundamental to fluid dynamics, and turbulence in particular is something that many people feel we could work with more effectively if we better understood how and why it's generated. The Navier-Stokes smoothness problem is an interesting and long-standing benchmark for this understanding; we don't know why it should be so hard to answer, so we hoped that the process of developing a proof to the problem would produce more understanding. (We may still be able to extract this understanding after the fact, if OpenAI's proof is fully human-comprehensible.)
Simply knowing that there exists a finite-time blowup is not practically useful. We know that fluids in the real world don't produce random singularities, so the result can't really have much physical meaning. What it illustrates is that the Navier-Stokes equations fail to model physical fluids in some yet to be characterized way.