QA cycles can be shortened. That is "merely" an engineering problem that can be solved: automation, parallelization, smarter tests, and better test plans. Not to mention better code review. There's a whole stack of books on continuous delivery & deployment out there in your library.
And even if the test cases were only run for, say, for every three upstream releases, I postulate that this would a) greatly reduce the chance that relevant regressions get introduced, b) even rev'ing the "enterprise" kernels every three upstream kernel releases would already be a huge boost over rev'ing them every 3 years.
I'd never have expected that, of all people, the _Linux_ folks would be the ones to claim that Linux/OSS can't work in an mission-critical environment but needs to be curtailed to a legacy "enterprise" model! I'm truly amazed.