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.