Are we really claiming that the only mechanism for orderly shutdown of the ACARS transmission is deliberate crew action? That the software/system does not provide for other paths (consistent with an accident or some failure scenario) leading to an orderly shutdown?
I would say it would take an in-depth knowledge of the system and its software for anyone to be able to make such a claim.
Other question: would it really come to mind to someone who wants to hide to shut down the ACARS? To shut that down first and then ten minutes later, the transponder? The opposite sequence would make so much more sense. Or both roughly at the same time.