You pretty obviously don't want to mess with it frivolously. But if there's something wrong with it, and you have to fix it? That seems better than the alternative where you can't. Note that the right to modify it doesn't imply that you're required to in the absence of any reason to.
Also, if someone wants to kill you in your sleep, they... don't need you to even have a pacemaker. And the security of medical devices is notoriously bad, so if you're worried about that sort of thing, be more worried that the status quo doesn't allow you to fix the existing remotely exploitable wireless security vulnerabilities.
That's just it though, in my opinion being able to flash the thing at all would count as a remotely exploitable wireless security vulnerability. The first thing I'd do if mine was flashable is lock it down to make sure it was no longer flashable. Does that make sense? I might not be articulating myself well here.
Removing the ability to flash it seems like a bad idea. Suppose they find a bug and release an official patch. You want it so you don't die, right? The alternative to flashing the one that's in you is that you need chest surgery again to replace it.
If it has a mechanism to flash it then they can give you the password for yours so that you can always do it yourself (or have someone do it) in the event that the manufacturer goes out of business before anyone finds the bug.
And if you really want to remove the ability to flash it, you could use your right to flash it to remove that feature, whereas the status quo is that it supports it -- insecurely -- and you aren't allowed to change it.
Also, if someone wants to kill you in your sleep, they... don't need you to even have a pacemaker. And the security of medical devices is notoriously bad, so if you're worried about that sort of thing, be more worried that the status quo doesn't allow you to fix the existing remotely exploitable wireless security vulnerabilities.