Surely they don't need backdoors when they can just exploit the awful network security that American networking equipment vendors already come with out of the box?
The US needed to smuggle Stuxnet in, but with networking equipment there's a treasure trove of shitty practices. Cisco and Juniper have been caught hiding hard-coded password how many times now?
At this point, any US company's products on software and hardware side can be safely considered an espionage asset. Even ignoring well known things like intercepting international packages during transit and putting malware into them.
Same goes obviously for ie Chinese stuff, but I don't think you guys realize how for outsider the border between China and US in terms of morality is practically non-existent now. I don't mean it in any snarky way, just looking at facts.
Also, China doesn't invade countries half around the world and bring them to utter destruction and misery for generations to come, killing thousands to millions of civilians and creating breeding grounds for things like ISIS. They do their own thing, quietly and patiently, with laser focus and for outsiders its at most 'not great not terrible' category.
Sometimes it's hard to tell if it's a real bug or a backdoor masquerading as a vulnerability.