The reality is that if Austin is granted waiver, then the presumption moving forward will be that waivers are customary. Already we see the question being framed as "Why give Mattis a waiver but not Austin?" Next time it will be "Why Mattis and Austin but not this person?" https://twitter.com/Susan_Hennessey/status/1336420337131663362
Instead of members of congress being expected to justify a vote for an extraordinary waiver, they will be expected to justify their refusals to grant waivers. Enshrining a norm in statute does nothing if Congress doesn't actually defend it.
The actual proposition Biden is advancing is that presidents should be able to pick the person they feel is the best candidate for the job and the Senate should confirm qualified candidates and securing waivers should be ordinary, not extraordinary.
It isn't a crazy position to take. ( I don't agree with it, but there are reasonable arguments.) But we (and Congress) should be clear eyed about the choice being made, which is not a vote to grant one waiver for one person, but instead an agreement to fundamentally alter a norm.
To be clear, the norm being altered here wouldn't be "civilian control of military." It would be the presumption of 7 years as minimum distance from service. But if the past four years have taught us anything, it's that these ecosystems are far more fragile than we recognized.