I suspect it's more a matter of the process to get the software certified being complex,
rather than the standards themselves.
For real high-integrity software, I think it would be very difficult to get Linux certified. A
big part of it is a verification of the development process, and you'd have an incredibly hard
time showing that Linux went through any sort of rigor (mainly because it didn't ;p). You
would probably have to take a snapshot of the kernel, rip out everything that isn't needed,
and then do code review and testing in-house, replacing or removing things that fail review or
don't conform to MISRA-C. By the time that's done whatever benefit there was to using Linux
over an in-house OS would likely be gone.
Does the process serve to inflate the profits of a company like Wind River? Probably, but then
again, Aerospace has a culture of safety that doesn't exist in most other areas where software
is developed and used. The fact that Avionics have been remarkably free of people-killing bugs
is a pretty good validation of the process.