Re: Not just about spying
A backdoor that shutdown the network is pretty useless though, unless it's used in a country that you're planning to militarily attack.
Think about it: If you're going to attack the country then the mobile networks going down would certainly cause disruption that could make your military attack more impactful, or likely to succeed. But if you're not attacking them then activating the hidden backdoor would realistically cause only some short-term public disruption - until the country were able fix it or switch to alternative hardware. They're certainly not going to back-down on any sanctions - the public anger alone would make this impossible - they're going to escalate sanctions and other actions instead.
China is absolutely not going to attack Germany, a country half-way around the world, NATO member, and key ally of three nuclear armed countries. So if there was one, then activating a hidden backdoor will only make things worse for China.
The only value of that kind of backdoor is as a threat _before_ the sanctions - but that's a one time threat, that would be just as likely to lead to the information being made public, and then every country will stop using Chinese equipment rather than just the US and whatever countries it can strong-arm into following suit.