The argument isn't that granting more freedoms to the owner of the device will magically make it more secure in all cases, for most it won't.

The argument is that removing freedoms from owners in the name of security is a false dichotomy because bad actors will still gain the ability to execute arbitrary code whilst owners of devices won't be able to do so.

Also, if I could provide the software I want to run, I'd probably not have iMessage.