If I was I was a intelligence agency in a country where there is a risk of blow-back, like in Europe or the US, I might prefer to use exploits. That way you haven't compromised your own country infrastructure (as much) and the risk of leaks is much lower since you dont have to work with an outside entity. A government agency forcing a domestic company to add backdoors, looks much worse if it gets out, then an agency using existing bugs.
If I'm a Intelligence agency in a country that doesn't care about blow-back like China or Russia, why bother finding and using an exploit, when you can call up the vendor and have them design the system with your use-case in mind? You dont have to worry about someone fixing the bug you have spent man years making exploitable, and you can make sure the backdoor can only be used by you. Its way more convenient and cost effective.
China is now suffering fron Huawei blowback in quite a few western countries.
I think Huawei, would have experienced almost the same blowback even if they didn't have any backdoor. Western intelligence experts, would have advised against using Huawei without any evidence of backdoors, simply because they know they would have put in backdoors if they where in the position of china, and they assume the Chinese aren't incompetent.
The blow back also serves a political purposes for everyone around. Western politicians/military gets to say "We need to protect ourselves against scary China!". And China's propaganda machine gets to say "Look at the terrible racist west treating us unfairly, by accusing us of bad things without evidence!".
- no need for backdoors since Huawei, Ericsson & Nokia are full to the brim with bugdoors (Huawei tops the chart here since many years already and as anyone involved in Inter-Operability-Testing (IOT) at the NEV will confirm).
- no need for "compromising networks" when you have the actual vendor (Huawei, Nokia, Ericsson often their subcontractors) sitting totally legally in your ISP's network and being paid for responding to the alarms raised and escalated by O&M.
- even the attacks against 3/4/5G become academic in the discussion of nation state threat actors when they can operate and exploit simply as an insider of the system. These weaknesses (as outrageous as they are) are useful but it's a different threat model