And the pictures of NSA employees opening parcels to modify the hardware before it gets shipped to certain targets immediatly comes to my mind.

That being said if you don’t control the software, modifying manually a handful of devices doesn’t scale. If you modify all of them the chance that you will be spotted is very high. If you control the software and it is encrypted / not readable, you can backdoor all devices of a whole country. So I can see how it is a step up in term of threat level.