Thank you for hanging in there, Eric. I mean no disrespect when
I insist on the question. I do have a point.

Taking your requirements one at a time:

-there is a reliable agent that can inspect the flight recorder for
signs of tampering

This is a question of physical security. The person responsible
is the official observer. This has always been true.

-it can answer critics with questions about the security of algorithms

Electronic security is not perfect. It can be "strong" or
"weak" just like physical security. If the world believes that
the electronic security designed into the flight recorder is
strong enough to do the job, then there will be no critics. If
the world believes that new techniques have rendered the flight
recorder vulnerable, then it is GFAC's responsibility to issue a
disapproval. The manufacturer need not exist.

-it can maintain the security keys

You may have to clarify this one. I don't think security
algorithms need maintenance.

So yes, this is the question (verbose version):

In the absence of any security challenge, criticism, disapproval
notice, or special procedure required of the manufacturer, would
the manufacturer's retirement be reason enough to cause the
automatic downgrading of a flight recorder from usable for world
records to unusable for world records?