[Blackthorn]

Is the medical device working right now? Yes. Could, upon upgrading, the device stop working, possibly in a subtle way that might kill somebody? Yes.

The approval process for medical devices is rightfully difficult. Software upgrades, even if they seem trivial, should not be a backdoor process of bypassing testing and approval.

[symtos]

...and could software deployed to the device by some random who just exploited some well-known security flaw that never got patched, kill people?

[Blackthorn]

This whole discussion has been about stuff air-gapped. That's not a guarantee when you have USB ports, but it does help.